French Onion Portobello Burgers

All the flavors of French onion soup are piled high on a tender portobello mushroom and sandwiched between English muffins. These are the ultimate meatless burgers!
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  • Level: Easy
  • Total: 35 min
  • Active: 35 min
  • Yield: 4 servings
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2 tablespoons butter

2 medium onions, thinly sliced

1 tablespoon plus 1 teaspoon fresh thyme leaves or 2 teaspoons dried thyme leaves or Italian seasoning blend

Kosher salt and freshly ground black pepper

1 tablespoon plus 1 teaspoon Worcestershire sauce

4 English muffins or hamburger buns, split

4 medium portobello mushroom caps (about 1 pound), stems removed

2 tablespoons olive oil or neutral oil

1/2 teaspoon garlic salt, or 1/8 teaspoon garlic powder and heaping 1/4 teaspoon kosher salt

1 cup shredded Gruyere, Swiss or Cheddar cheese (about 4 ounces)

1/3 cup mayonnaise

1 clove garlic, finely grated

1/2 cup crispy fried onions, such as French's (or crushed corn chips, pretzels or potato chips)


  1. Preheat the broiler. Line a rimmed baking sheet with aluminum foil.
  2. Melt the butter in a medium skillet over medium-low heat. Add the sliced onions, 1 tablespoon fresh thyme or 1 1/2 teaspoons dried, 1/2 teaspoon kosher salt and a few grinds of pepper. Cook, stirring occasionally, until the onions are deep golden brown and caramelized, about 15 minutes. If the skillet gets too brown, add 2 tablespoons water and scrape the brown bits from the bottom of the skillet with a wooden spoon; continue to cook until all the water has evaporated. Add 1 tablespoon of the Worcestershire and stir until completely absorbed, about 1 minute more.
  3. Meanwhile, place the muffins or buns on the prepared baking sheet and broil until toasted and browned along the edges, 1 to 2 minutes. Set aside for serving.
  4. Put the mushroom caps, gill-side up, on the same baking sheet. Broil until they release their juices and soften slightly, about 6 minutes. Flip the mushrooms, brush the tops with the oil and season with the garlic salt. Broil until lightly browned and the juices have cooked off, about 6 minutes more. Top each mushroom with 1/4 cup cheese, then return to the broiler until the cheese melts, 1 to 1 1/2 minutes.
  5. While the mushrooms are broiling, stir together the mayonnaise, garlic, remaining 1 teaspoon fresh thyme or 1/2 teaspoon dried thyme and 1 teaspoon Worcestershire in a small bowl until smooth. Season with a pinch of kosher salt and pepper. Spread on the inside of the toasted muffin or bun tops. Place 1 portobello mushroom on each muffin or bun bottom. Layer with the caramelized onions and crispy fried onions, then close the burgers and serve.
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