Roasted Portobello Mushroom Cheeseburgers with Caramelized Onions and Pimento Aioli

Several of my friends are vegetarians, so I came up with this recipe last summer when I was having a barbecue. They are so good, even carnivores will devour them. If you can find Portuguese muffins, their sweet flavor makes for a delicious bun option.
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  • Level: Intermediate
  • Total: 3 hr 10 min
  • Prep: 30 min
  • Inactive: 2 hr
  • Cook: 40 min
  • Yield: 6 servings
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For the burgers:

Juice of 1 lemon (2 to 3 tablespoons)

1/4 cup extra virgin olive oil

2 tablespoons worcestershire sauce

2 teaspoons fresh thyme leaves

1/4 teaspoon garlic powder

6 portobello mushrooms, wiped clean and stems removed 

6 portobello mushrooms, wiped clean and stems removed

Kosher salt and freshly ground black pepper

6 slices Muenster cheese (or Swiss, white cheddar, provolone-your choice)

6 Portuguese muffins or brioche hamburger buns, buttered and toasted

1 1/2 cups loosely packed arugula

For the onions:

1 tablespoon olive oil

1 medium Vidalia onion, thinly sliced

1/4 cup water

Kosher salt and freshly ground pepper

For the aioli:

1/2 cup mayonnaise

2 tablespoons jarred chopped pimentos, drained

1 tablespoon red wine vinegar

1 garlic clove, minced

1 tablespoon minced flat-leaf parsley

Kosher salt and freshly ground black pepper


  1. For the burgers: In a shallow baking dish, use a whisk to combine the lemon juice, olive oil, Worcestershire, thyme, and garlic powder. Place mushrooms in the marinade, cover, and refrigerate for at least 1 hour. Turn over the mushrooms, cover again, and refrigerate at least another hour. (Mushrooms can marinate for several hours.)
  2. For the onions: In the meantime, make the onions. In a skillet, heat the olive oil over medium heat. Add the onions and cook about 5 minutes, stirring occasionally. Add the water, cover, and reduce heat to low. Cook 20 to 30 minutes, stirring occasionally, until the onions are golden brown. Remove the pan from the heat. Season the onions with salt and pepper. Set aside.
  3. For the aioli: Combine all ingredients in a small bowl. Refrigerate until serving time.
  4. To finish the burgers: Preheat oven to 450 degrees F. Remove mushrooms from marinade and season with salt and pepper on both sides. Place the mushrooms on a baking sheet, gill sides up. Bake for 5 minutes. Remove from the oven and arrange cheese and caramelized onions on the gills of each mushroom and bake another 5 minutes, until the cheese is melted and bubbly. Place 1 mushroom on each muffin and garnish with aioli and arugula. Serve immediately.
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