Portobello French Dip with Horseradish Aioli

BY Emily Carter January 2, 2026

Plant-forward eating often focuses on what is missing, but this Portobello French Dip with Horseradish Aioli shifts the narrative toward abundance and texture. By utilizing the naturally dense, meaty quality of portobello caps, this sandwich provides a satisfying weight that rivals any traditional beef-based counterpart. The combination of slow-browned onions, a sharp garlic-infused spread, and a deeply savory dipping broth transforms simple pantry staples into a sophisticated meal. This recipe honors the classic American bistro tradition while proving that mushrooms can easily command the center of the plate when treated with the right roasting techniques and seasonings.

Table of Contents

Ingredients

Roasted Mushrooms

4 large portobello mushroom caps, sliced into thin strips
2 to 3 tablespoons olive oil
1 to 2 teaspoons Montreal steak seasoning (or salt and pepper)

Caramelized Onions and Au Jus

2 tablespoons butter
1 large onion, thinly sliced
1/4 cup white wine
1 tablespoon Worcestershire sauce (omit for strictly vegetarian or vegan versions)
1 tablespoon soy sauce
2 cups vegetable or beef broth
Salt, pepper, and a pinch of sugar to taste

Horseradish Aioli

1/2 cup mayonnaise
1 teaspoon prepared horseradish
1 clove garlic, grated
Pinch of salt

French Dip Essentials

4 crusty sourdough rolls or a large baguette cut into four sections
Provolone cheese slices

Instructions

Roast the Mushrooms: Set your oven to four hundred fifty Fahrenheit. Arrange the mushroom strips on a baking sheet, tossing thoroughly with the olive oil and steak seasoning. Roast for twenty to thirty minutes.
Sauté the Onions: While the mushrooms roast, melt butter in a large skillet over medium heat. Add the sliced onions and cook until they soften.
Caramelize: Reduce the heat to low and continue cooking the onions for approximately twenty minutes. Stir occasionally until they reach a deep golden brown.
Deglaze and Simmer: Pour the white wine into the skillet with the onions, allowing it to bubble and reduce. Stir in the Worcestershire sauce, soy sauce, and broth.
Separate the Components: Allow the liquid to simmer until it has reduced slightly. Using tongs, lift the caramelized onions out of the broth and set them aside. Taste the remaining au jus and adjust seasoning.
Mix the Aioli: In a small bowl, whisk together the mayonnaise, horseradish, grated garlic, and salt.
Assemble and Toast: Split the rolls and butter them if desired. Layer the roasted mushrooms onto the bread and top with slices of Provolone. Place the sandwiches back in the oven for about five minutes until the cheese is bubbling.
Finish and Serve: Open the toasted sandwiches to add a portion of the reserved caramelized onions and a generous dollop of the horseradish aioli. Serve immediately with a small bowl of the hot au jus for dipping.

Step-By-Step Details

Roasting for Texture

The initial step of roasting the mushrooms at four hundred fifty Fahrenheit is vital for moisture removal. Portobellos contain a high volume of water; by roasting them at a high temperature, you evaporate that moisture, leaving behind a concentrated, chewy texture. Look for the edges of the strips to become slightly crisp and dark. If the mushrooms look soggy, they likely need another five minutes in the oven.

The Art of the Onion

Caramelization is a lesson in patience. When you turn the heat down after the initial softening, you are encouraging the natural sugars in the onion to brown without burning. If the pan looks too dry or the onions are sticking, add a tiny splash of water. The final result should be jam-like and intensely sweet, providing a counterpoint to the salty broth.

Building the Au Jus

When you add the wine to the pan, use a wooden spoon to scrape up any browned bits (fond) from the bottom of the skillet. This is where the depth of flavor lives. The combination of soy sauce and Worcestershire provides “umami,” the savory fifth taste that gives the broth its “meaty” backbone. Simmering the liquid for those few extra minutes concentrates these flavors so the dip doesn’t taste like plain broth.

The Final Toast

Don’t skip the second trip to the oven. Melting the Provolone directly onto the mushrooms helps bind the sandwich together. You want the bread to be sturdy enough to withstand being dunked into the broth, so a light toast is essential for structural integrity. The cheese acts as a moisture barrier, preventing the bread from getting soggy the moment the onions and aioli are added.

Pro Tips

Scrape the Gills: If you prefer a cleaner-looking sandwich and a lighter broth, you can use a spoon to gently scrape the dark gills out of the underside of the portobello caps before slicing.¹
Even Slices: Try to slice the onions as uniformly as possible. This ensures they all caramelize at the same rate, preventing small pieces from burning while thicker pieces remain raw.
Fresh Garlic Matters: For the aioli, use a microplane to grate the garlic into a paste.² This ensures the garlic flavor is distributed evenly throughout the mayo without any harsh, crunchy bits.
Wine Choice: Use a dry white wine like Sauvignon Blanc or Pinot Grigio. Avoid anything sweet, as the caramelized onions already provide plenty of sugar to the profile.
Temperature Control: If your oven has a “convection” setting, use it for the mushrooms to help them crisp up even faster, but keep an eye on them to prevent charring.
The Bread Test: Choose a roll that feels crusty on the outside but has a soft crumb. If the bread is too soft (like a brioche bun), it will fall apart during the dipping process.

Variations or Substitutions

Vegan Version: Swap the butter for olive oil or a plant-based margarine. Use a vegan mayonnaise for the aioli and ensure your Worcestershire sauce is a vegan-certified brand (which omits anchovies). Replace the Provolone with a cashew-based smoked gouda or simply omit the cheese.
Gluten-Free Modification: Use a high-quality gluten-free baguette. Ensure your soy sauce is replaced with tamari and verify that your Worcestershire sauce is gluten-free.
Mushroom Variety: While portobellos provide the best “steak” texture, you can use a mix of cremini (baby bella) and shiitake mushrooms for a more varied earthy flavor profile.
Herb Infusion: Add a sprig of fresh thyme or a bay leaf to the au jus while it simmers to add a woody, herbal dimension to the dipping sauce.

Serving Suggestions

Classic Bistro Style: Serve the sandwich alongside a handful of kettle-cooked potato chips and a crisp dill pickle spear to lean into the deli aesthetic.
Light Pairing: A simple arugula salad with a lemon vinaigrette provides a peppery brightness that cuts through the richness of the cheese and the aioli.
Double the Dip: Provide extra au jus in wide, shallow ramekins. This makes it easier to submerge the entire surface of the sandwich without the liquid overflowing.

FAQs

Can I make the au jus in advance?

Yes, the broth and caramelized onions can be prepared up to two days in advance. Simply reheat the liquid on the stovetop before serving. The mushrooms are best prepared fresh to maintain their texture.

What if I don’t have Montreal steak seasoning?

You can create a quick substitute by mixing equal parts coarse salt, cracked black pepper, garlic powder, and a pinch of crushed red pepper flakes.

Why is my au jus too salty?

The saltiness can vary depending on the brand of broth and soy sauce used. If it tastes too salty after simmering, add a splash of water or a little more white wine to balance the levels.

Can I use a different cheese?

While Provolone is traditional for its mild flavor and excellent meltability, Swiss cheese, Gruyère, or even a sharp white cheddar would work well in this Portobello French Dip with Horseradish Aioli.

Nutrition Information

Nutrient	Amount Per Serving
Calories	485 kcal
Total Fat	28g
Saturated Fat	11g
Cholesterol	35mg
Sodium	1150mg
Total Carbohydrates	42g
Dietary Fiber	4g
Sugars	8g
Protein	16g

Disclaimer

Nutrition information is an estimate and may vary based on preparation and ingredients used.