Portobello French Dip with Horseradish Aioli

Portobello French Dip with Horseradish Aioli is a bold and satisfying vegetarian twist on the classic French dip sandwich. Thick, meaty portobello mushrooms are roasted until tender and juicy, layered on toasted rolls with melty cheese, and topped with zippy horseradish aioli. Served with a savory homemade au jus for dipping, this sandwich delivers big flavor and hearty comfort—without any meat.

Why You’ll Love This Recipe

• Hearty and meaty, even without meat
• Packed with bold, savory, umami-rich flavor
• Creamy, spicy horseradish aioli adds the perfect kick
• Great for vegetarians and mushroom lovers
• Ideal for lunch, dinner, or a weekend indulgence

Ingredients

(Tip: You’ll find the full list of ingredients and measurements in the recipe card below.)

For the roasted mushrooms:

• 4 large portobello mushroom caps, sliced
• 2 tablespoons olive oil
• 1 teaspoon Montreal steak seasoning or salt and pepper to taste

For the caramelized onion au jus:

• 1 tablespoon butter
• 1 tablespoon olive oil
• 1 large onion, thinly sliced
• 2 cups vegetable broth (or beef-style vegetarian broth)
• 1 tablespoon soy sauce
• 1 tablespoon Worcestershire sauce (or vegan alternative)
• ¼ cup dry white wine (optional)
• Salt and pepper to taste

For the horseradish aioli:

• ½ cup mayonnaise (or vegan mayo)
• 1 tablespoon prepared horseradish
• 1 teaspoon Dijon mustard
• 1 small garlic clove, finely grated
• Pinch of salt

For the sandwiches:

• 4 crusty sandwich rolls or baguette sections
• 4 slices Provolone, Swiss, or vegan cheese
• Optional: fresh arugula or baby spinach

Directions

1. Roast the mushrooms:
   Preheat oven to 425°F (220°C). Toss mushroom slices with olive oil and seasoning. Spread on a baking sheet and roast for 20–25 minutes, flipping once halfway through, until tender and browned.
2. Make the au jus:
   In a saucepan, heat butter and oil over medium heat. Add sliced onions and cook until golden and soft, about 15–20 minutes. Stir in wine (if using), soy sauce, Worcestershire, and broth. Simmer for 5–10 minutes to blend flavors. Strain and reserve the broth for dipping; set onions aside.
3. Prepare the aioli:
   In a small bowl, mix mayonnaise, horseradish, mustard, garlic, and a pinch of salt. Refrigerate until ready to use.
4. Assemble the sandwiches:
   Toast the rolls if desired. Spread a layer of horseradish aioli on the inside of each roll. Layer on the roasted mushrooms, caramelized onions, and cheese. Optionally, add arugula or baby greens.
5. Melt the cheese (optional):
   Place the sandwiches open-faced under the broiler or in a hot oven for a few minutes until the cheese melts.
6. Serve:
   Slice in half and serve with warm au jus on the side for dipping.

Servings and Timing

• Servings: 4 sandwiches
• Prep Time: 20 minutes
• Cook Time: 30 minutes
• Total Time: 50 minutes

Variations

• Vegan version: Use vegan mayo, vegan Worcestershire sauce, and plant-based cheese
• Spicy: Add a pinch of cayenne or sriracha to the aioli
• Cheese swap: Try sharp cheddar, mozzarella, or smoked gouda
• Add-ins: Sautéed bell peppers, roasted garlic, or a layer of spinach

Storage/Reheating

• Refrigerate: Store the components separately for up to 3 days
• Reheat: Reheat mushrooms and onions in a skillet; toast assembled sandwich briefly in oven to melt cheese
• Freezing: Not recommended due to texture changes in mushrooms and aioli

FAQs

Can I use other mushrooms?

Yes! Cremini, shiitake, or oyster mushrooms also work well, but portobellos give the best meaty texture.

Is horseradish aioli spicy?

It has a mild kick. Adjust the horseradish amount to your taste.

What’s the best bread for this?

Use crusty rolls, baguette sections, or ciabatta for a sturdy base that holds up to dipping.

Can I make the aioli in advance?

Yes, it can be made up to 3 days ahead and stored in the fridge.

What makes this dip vegetarian?

Using vegetable broth and a vegetarian Worcestershire sauce keeps the recipe meat-free.

Do I need to strain the au jus?

Straining isn’t required but helps if you want a smooth dip without onion bits.

Can I skip the cheese?

Absolutely. The sandwich is still delicious without it or with a dairy-free version.

Can I prep this for a party?

Yes! Roast the mushrooms, make the aioli and au jus ahead of time. Assemble and warm just before serving.

How do I get the best caramelized onions?

Cook low and slow—don’t rush! Stir occasionally and give them time to develop color.

Can I make it gluten-free?

Use gluten-free rolls and check that your Worcestershire and soy sauce are gluten-free.

Conclusion

Portobello French Dip with Horseradish Aioli is a bold, satisfying sandwich that brings together deep savory flavors and creamy, spicy notes in one delicious bite. Whether you’re vegetarian, looking to eat less meat, or just love mushrooms, this French dip is a crave-worthy alternative to the classic beef version. It’s the kind of sandwich that turns a simple meal into something special.

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