Dinner

Portobello French Dip with Horseradish Aioli Recipe

ByMariam Harutyunyan February 11, 2026February 11, 2026

Introduction

Portobello French Dip with Horseradish Aioli is a flavorful vegetarian twist on the classic sandwich. Roasted mushrooms, caramelized onions, and melted provolone come together for a hearty meal perfect for dipping in savory au jus.

A close-up side view of a sandwich with four main layers is shown. The top layer is a light golden toasted bun that looks soft and slightly crunchy. Below it, there is a layer of caramelized onions with a shiny brown color and slightly translucent texture. Under the onions, there are several dark brown grilled mushroom slices adding a rich texture and color. The next layer is creamy white cheese melted softly. The sandwich sits on a piece of white parchment on top of a textured wooden board with visible grain. The background is a white marbled texture. photo taken with an iphone --ar 4:5 --v 7

Ingredients

4 portobello mushroom caps, sliced into thin strips
2-3 tablespoons olive oil

1-2 teaspoons Montreal steak seasoning (or just a sprinkle of salt and pepper)
2 tablespoons butter

1 large onion, thinly sliced
1/4 cup white wine

1 tablespoon Worcestershire (omit for vegetarian/vegan)
1 tablespoon soy sauce
2 cups beef or vegetable broth

Salt, pepper, and a pinch of sugar to taste
1/2 cup mayo
1 teaspoon horseradish

1 clove garlic, grated
Pinch of salt
4 crusty sourdough rolls, or a baguette cut into 4 sections

Provolone cheese slices

Instructions

Step 1: Preheat your oven to 450°F (230°C). Toss the sliced portobello mushrooms with olive oil and Montreal steak seasoning (or salt and pepper). Spread them on a roasting pan and roast for 20–30 minutes until tender and slightly caramelized.
Step 2: While the mushrooms roast, melt butter in a large skillet over medium heat. Add the thinly sliced onions and sauté until softened. Lower the heat and cook, stirring occasionally, for about 20 minutes until the onions are deeply golden and caramelized.

Step 3: Pour white wine into the skillet with the onions and let it sizzle until mostly evaporated. Stir in Worcestershire sauce, soy sauce, and broth. Simmer gently until the liquid reduces slightly. Remove the onions with tongs and set them aside. Season the au jus with salt, pepper, and a pinch of sugar to taste.
Step 4: Prepare the horseradish aioli by mixing mayo, horseradish, grated garlic, and a pinch of salt in a small bowl. Adjust the horseradish amount to your preferred spice level.
Step 5: If desired, butter the cut rolls or baguette sections. Pile the roasted mushrooms on the bread, then top with provolone slices. Place the sandwiches on the oven’s roasting pan and bake for about 5 minutes or until the cheese melts and the bread is toasted.

Step 6: Remove sandwiches from the oven, top with caramelized onions, and add a dollop of horseradish aioli. Serve immediately with the warm au jus for dipping.

Tips & Variations

For a vegan version, omit the cheese and Worcestershire sauce, and use vegan mayo in the aioli.
Try adding fresh thyme or rosemary to the mushrooms before roasting for added aroma.

If you prefer a spicier aioli, add a pinch of cayenne pepper or hot sauce.
Use crusty bread like ciabatta if you want a different texture for your sandwich.

Storage

Store any leftover mushroom strips and caramelized onions separately in airtight containers in the refrigerator for up to 3 days. Assemble sandwiches fresh to maintain bread texture. Reheat mushrooms and onions gently on the stove or in the microwave. The horseradish aioli keeps well refrigerated for up to one week.

How to Serve

A close-up of a sandwich being dipped into a bowl of brown sauce. The sandwich has a golden brown toasted bun with two layers: the bottom layer holds dark brown grilled sausage pieces topped with soft caramelized onions in light golden color, and the top layer is spread with a creamy white sauce speckled with black pepper. The bowl holding the sauce is white, and the background shows a smooth white marbled texture. Photo taken with an iphone --ar 4:5 --v 7

Serve this delicious recipe with your favorite sides.

FAQs

Can I use other mushrooms instead of portobello?

Yes, cremini or shiitake mushrooms work well if sliced thinly and roasted similarly. They may have slightly different textures and flavors but will still be delicious.

What can I substitute for Worcestershire sauce?

If avoiding Worcestershire sauce, especially for vegan diets, omit it or replace with a splash of tamari or extra soy sauce to keep the umami flavor.

Mariam Harutyunyan