Spain's famous piquillo peppers add a sweet-spicy edge to the aioli. Idiazábal, a smoky sheep's-milk cheese from the Basque region, is great with the portobellos. If you can't find Idiazábal, substitute smoked Gouda.
Recipe information
Yield
Makes 6
Ingredients
Aioli:
Mushrooms:
Preparation
For aioli:
Step 1
Puree first 4 ingredients in mini processor. Transfer to small bowl. Whisk in mayonnaise; season with salt, black pepper, and cayenne pepper. DO AHEAD: Can be made 3 days ahead. Cover; chill.
For mushrooms:
Step 2
Arrange mushroom caps and onion slices on baking sheet; brush both sides with oil, then sprinkle with salt, pepper, and thyme. Turn mushrooms, gill side up, and sprinkle with garlic. Let stand at least 30 minutes and up to 2 hours.
Step 3
Prepare barbecue (medium-high heat). Place mushrooms, gill side up, and onion slices on rack. Cook until grill marks appear, about 4 minutes. Turn vegetables over, transferring to cooler part of grill. Cover and cook until mushrooms are very tender and onion slices still retain some texture, about 8 minutes. Turn mushrooms, gill side up; top with cheese. Arrange rolls, cut side down, on grill. Cover; cook until cheese on mushrooms melts and cut sides of rolls are golden, about 1 minute longer.
Step 4
Arrange roll bottoms on plates; spread with aioli. Top each with mushroom, onion slice, and watercress. Spread cut side of roll tops with aioli; press onto watercress.