These meaty grilled portobello burgers are one of our favorite vegetarian meals. They're flavorful and delicious, stacked with sweet bell peppers, grilled onions, melty cheese and an irresistible garlic aioli.
Clean the mushroom caps by brushing away the debris from the caps with a damp paper towel. Remove the stems with your fingers, or slice them off with a sharp knife and discard.
Use a spoon to scrape the gills from the underside of the mushroom caps. Set aside.
FOR THE SPICE RUB:
In a small bowl combine the garlic, thyme, tarragon, rosemary, crushed red pepper flakes, kosher salt and black pepper.
Brush the tops and bottoms of each mushroom cap with 1 tablespoon of the olive oil and divide the spice rub between them, rubbing it into the mushrooms (the seasoning will stick better to the underside of the fungi).
Brush the sliced onions with the remaining olive oil and set aside.
FOR PORTOBELLO MUSHROOM SANDWICH
Heat a grill to a medium high heat - about 375-400 degrees.
Place the mushrooms on the grill, cap side down and cook for 4-5 minutes. Grill the red onions at the same time. Flip the mushrooms and onions and cook for an additional 3-4 minutes. until the flesh is softened and exterior takes on some grill marks.
Top each mushroom with a slice of cheese and cook for 1-2 more minutes until the cheese is melty.
Split the buns in half and toast them on the grill for 1-2 minutes, until lightly browned.
ASSEMBLE THE PORTOBELLO BURGERS:
Spread garlic aioli on the top and bottom buns. Place mushrooms on the buns. Top with one or two slices of roasted bell peppers, grilled onions and micro greens or lettuce. Place the top bun on the mushroom burger and serve.