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A picture of Portobello French Dip with Horseradish Aioli

Portobello French Dip with Horseradish Aioli

  • Author: Lindsay
  • Total Time: 45 minutes
  • Yield: 4 mega sandwiches 1x


These Portobello French Dips are about to change your life! Roasty portobello strips, golden caramelized onions, melted Provolone cheese, zippy horseradish aioli, all piled on a warm, crusty roll.



Roasted Mushrooms

  • 4 portobello mushroom caps, sliced into thin strips
  • 23 tablespoons olive oil
  • 12 teaspoons Montreal steak seasoning (or just 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 (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 mayo
  • 1 teaspoon 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


  1. Roast the Mushrooms: Preheat oven to 450 degrees. Toss mushroom slices with oil and seasoning. Roast for 20-30 minutes. 
  2. Caramelize the Onions: Meanwhile, melt the butter in a large skillet over medium heat. Add onions; saute until softened, then turn the heat down to caramelize the onions for 20 minutes or so, stirring occasionally until they are deep golden brown.
  3. Make the Au Jus: Add the wine to the onions in the pan. Let it sizzle out. Add the Worcestershire and soy sauce. Add the broth. Simmer until reduced slightly. Pull out the onions with tongs and set aside. Season the au jus to taste.
  4. Toast the French Dips: Butter the bread if you want. Pile mushrooms on the bread. Cover with cheese. Return to the oven on the same roasting pan for about 5 minutes (enough to toast the bread and melt the cheese). 
  5. Feast: Top the sandwiches with caramelized onions. Give em each a dollop of horseradish aioli. Serve with the au jus for dipping! HOLY MOLY that’s good.


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

Keywords: portobello mushrooms, french dip, vegetarian sandwich, caramelized onions