Portobello French Dip with Horseradish Aioli

Enjoy a vegetarian twist on the classic French Dip with roasted portobello mushrooms, golden caramelized onions, melted Provolone cheese, and zesty horseradish aioli, all stacked on a warm, crusty roll. Dunk it into rich, flavorful au jus, and you’ve got a satisfying, meatless comfort sandwich that’s perfect for any meal.

Kids and adults alike will love this—one 4-year-old couldn’t stop eating and declared he would want it every day! This recipe is part of our Plant-Powered January series, making vegetables fun, delicious, and irresistible.

Ingredients

Roasted Mushrooms

• 4 portobello mushroom caps, sliced thin
• 2–3 tbsp olive oil
• 1–2 tsp Montreal steak seasoning (or salt and pepper)

Caramelized Onions & Au Jus

• 2 tbsp butter
• 1 large onion, thinly sliced
• 1/4 cup white wine (or water)
• 1 tbsp Worcestershire sauce (omit for vegan)
• 1 tbsp soy sauce
• 2 cups vegetable or beef broth
• Salt, pepper, and a pinch of sugar

Horseradish Aioli

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

French Dip Essentials

• 4 crusty sourdough rolls or a baguette, cut into 4 sections
• Provolone cheese slices

Instructions

Roast the Mushrooms

1. Preheat oven to 450°F (230°C).
2. Toss mushroom strips with olive oil and seasoning.
3. Roast for 20–30 minutes until tender and lightly browned.

Caramelize the Onions

1. Melt butter in a skillet over medium heat.
2. Add onions and sauté until softened.
3. Lower heat and cook for 20 minutes or until onions are golden and caramelized, stirring occasionally.

Prepare the Au Jus

1. Pour white wine into the onions; allow it to sizzle off.
2. Stir in Worcestershire and soy sauce.
3. Add broth and simmer until slightly reduced.
4. Remove onions and set aside; adjust seasoning as needed.

Assemble the Sandwich

1. Optionally butter the rolls.
2. Layer roasted mushrooms on the bread and top with Provolone.
3. Toast in the oven for 5 minutes until cheese melts and bread is crisp.
4. Add caramelized onions and a dollop of horseradish aioli.
5. Serve with au jus for dipping.

Tips & Variations

• Worcestershire alternative: Use vegan Worcestershire sauce or soy sauce.
• White wine substitute: Replace with water or vegetable broth.
• Gluten-free option: Use gluten-free baguette or rolls and tamari for soy sauce.

Prep Time: 15 minutes
Cook Time: 30 minutes
Category: Sandwich
Cuisine: American
Method: Oven

This Portobello French Dip is the ultimate vegetarian comfort sandwich—crispy, cheesy, and full of flavor. Perfect for lunch, dinner, or a cozy weekend treat!