Portabella Mushroom Burgers with Red Pepper Mayonnaise

Step 1
Place mushrooms into a large resealable plastic bag; add soy sauce, balsamic vinegar, and garlic. Seal bag and shake to combine ingredients; marinate mushrooms at room temperature for 2 to 3 hours, turning bag occasionally.

Step 2
Preheat grill for medium-high heat and lightly oil the grate.

Step 3
Place red bell pepper onto the grate and grill until the skin is blistered and charred, turning often, about 10 minutes. Transfer pepper to a bowl and cover with plastic wrap; let pepper cool. When pepper is cool enough to handle, remove and discard skin, stem and seed the pepper, and chop.

Step 4
Mix pepper into mayonnaise in a bowl and stir in cayenne pepper. Chill red pepper mayonnaise until serving time.

Step 5
Grill marinated mushroom caps until browned and juicy, about 6 minutes per side. Serve mushrooms on buns, topped with red pepper mayonnaise.