Introduction

This Portobello French Dip with Horseradish Aioli is a flavorful vegetarian twist on the classic sandwich. Tender roasted mushrooms, sweet caramelized onions, and melty provolone come together with a tangy aioli and savory au jus for dipping. It’s an irresistible meal perfect for any occasion.

A sandwich on a white bread roll with a golden toasted crust. The bottom layer shows a thick spread of creamy white sauce, topped by a layer of dark, glossy grilled mushrooms. Above the mushrooms is a generous amount of caramelized onions with a slightly shiny texture, and a slice of melted white cheese peeks out from under the top bread layer. The sandwich is placed on white parchment paper resting on a wooden cutting board with a natural grain pattern, all set against a white marbled surface background. 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 (for aioli)
4 crusty sourdough rolls, or a baguette cut into 4 sections
Provolone cheese slices

Instructions

Step 1: Preheat the oven to 450°F (230°C). Toss the sliced mushrooms with olive oil and Montreal steak seasoning (or salt and pepper). Spread them on a baking sheet and roast for 20–30 minutes until tender and caramelized.
Step 2: While the mushrooms roast, melt the butter in a large skillet over medium heat. Add the sliced onions and sauté until softened. Reduce heat and cook gently for about 20 minutes, stirring occasionally, until the onions are deeply golden brown and caramelized.
Step 3: Add the white wine to the caramelized onions and allow it to sizzle and reduce. Stir in Worcestershire sauce and soy sauce, then pour in the 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 taste.
Step 4: Prepare the horseradish aioli by mixing together mayo, horseradish, grated garlic, and a pinch of salt in a small bowl. Set aside.
Step 5: If desired, lightly butter the rolls or baguette sections. Layer the roasted mushrooms on the bread, then cover with provolone cheese slices. Place the sandwiches back in the oven on the baking sheet for about 5 minutes, just long enough to toast the bread and melt the cheese.
Step 6: Assemble the sandwiches by topping the melted cheese and mushrooms with the caramelized onions. Add a generous dollop of horseradish aioli on each. Serve immediately with the warm au jus on the side for dipping.

Tips & Variations

Use vegetable broth and omit Worcestershire sauce to make this fully vegan; substitute mayo with vegan mayo for the aioli.
Try adding fresh thyme or rosemary to the mushrooms before roasting for an extra herbaceous note.
For a spicier kick, mix a little hot sauce into the horseradish aioli.
To get a crispier bread crust, toast the sandwiches under a broiler for 1–2 minutes instead of the oven.

Storage

Store leftover components separately in airtight containers in the refrigerator for up to 3 days. Reheat mushrooms, onions, and au jus gently on the stove. Assemble sandwiches fresh for best texture. The horseradish aioli can be kept refrigerated for up to 5 days.

How to Serve

The image shows a close-up of a sandwich being held by a woman's hand, dipped slightly into a white bowl filled with dark brown sauce. The sandwich has a soft, light brown bun that is split open to reveal multiple layers inside: crispy bacon with a reddish-brown color, melted white cheese, and cooked onions that are golden and slightly translucent. The background and surface are a white marbled texture, giving a clean and bright look to the scene. 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 as they have a meaty texture and rich flavor. Just adjust roasting time as needed.

How do I make this recipe gluten-free?

Use gluten-free bread and ensure your soy sauce is gluten-free (tamari is a good option). Also, check that Worcestershire sauce is gluten-free or omit it.