Description

This Portobello French Dip with Horseradish Aioli is a flavorful vegetarian twist on the classic French dip sandwich. Roasted portobello mushrooms are paired with caramelized onions and melted provolone cheese on crusty sourdough rolls, served alongside a rich and savory au jus broth for dipping. The sandwich is finished with a zesty horseradish aioli for an extra kick, making it a hearty and satisfying meal perfect for lunch or dinner.

Ingredients

Scale

Mushrooms and Seasoning

4 portobello mushroom caps, sliced into thin strips
2–3 tablespoons olive oil
1–2 teaspoons Montreal steak seasoning or a sprinkle of 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 vegetarian/vegan)
1 tablespoon soy sauce
2 cups beef or vegetable broth
Salt, pepper, and a pinch of sugar to taste

Horseradish Aioli

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

Sandwich Assembly

4 crusty sourdough rolls or a baguette cut into 4 sections
Provolone cheese slices
Butter for bread (optional)

Instructions

Roast the Mushrooms: Preheat your oven to 450°F (232°C). Toss the sliced portobello mushrooms with olive oil and Montreal steak seasoning (or salt and pepper). Spread them out on a baking sheet and roast for 20-30 minutes until tender and slightly caramelized.
Caramelize the Onions: While the mushrooms roast, melt the butter in a large skillet over medium heat. Add the thinly sliced onions and sauté until softened. Reduce the heat to low and continue cooking the onions, stirring occasionally, for about 20 minutes until they become deep golden brown and caramelized.
Make the Au Jus: Pour the white wine into the skillet with the onions and let it cook down until the alcohol sizzles off. Stir in Worcestershire sauce and soy sauce, then add broth. Simmer gently until the liquid reduces slightly. Use tongs to remove the onions and set them aside. Season the au jus with salt, pepper, and a pinch of sugar to balance the flavors.
Prepare the Horseradish Aioli: In a small bowl, combine mayonnaise, horseradish, grated garlic, and a pinch of salt. Mix well and set aside until ready to serve.
Toast and Assemble the Sandwiches: Optionally butter the inside of your sourdough rolls or baguette sections. Pile the roasted mushrooms onto the bread, then top with provolone cheese slices. Place the sandwiches on the same roasting pan used for the mushrooms and return to the oven for about 5 minutes, just long enough to toast the bread and melt the cheese.
Serve: Remove the sandwiches from the oven and top them generously with the caramelized onions. Add a dollop of the horseradish aioli on each sandwich. Serve immediately with the warm au jus on the side for dipping. Enjoy the rich, savory flavors and delightful textures of this vegetarian French dip!

Notes

For a vegan version, omit the Worcestershire sauce, replace mayonnaise with vegan mayo, and use a plant-based cheese or omit cheese entirely.
The au jus can be made ahead and reheated gently before serving.
If you prefer a stronger horseradish flavor, add more horseradish to the aioli to taste.
Using vegetable broth instead of beef broth keeps this recipe vegetarian-friendly.
Be careful not to over-roast the mushrooms—they should be tender but not dried out.

Prep Time: 15 minutes
Cook Time: 45 minutes
Category: Sandwich
Method: Baking
Cuisine: American

Keywords: Portobello French Dip, Vegetarian French Dip, Mushroom Sandwich, Horseradish Aioli, Caramelized Onions, Au Jus, Sourdough Sandwich