GRILLED PORTOBELLO BURGERS WITH PIQUILLO PEPPER AïOLI AND WATERCRESS

Grilled Portobello Burgers with Piquillo Pepper Aïoli and Watercress image

Provided by Tony Rosenfeld

Yield Makes 6

Number Of Ingredients 16

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Aioli:
1/2 cup chopped drained piquillo peppers or roasted red peppers from jar
1 tablespoon Sherry wine vinegar
2 teaspoons finely chopped fresh thyme
1 garlic clove, minced
1/2 cup purchased mayonnaise
Cayenne pepper
Mushrooms:
6 large portobello mushrooms, stemmed, dark gills scraped out
1 sweet onion (such as Vidalia or Walla Walla), cut into 1/3-inch-thick slices
Extra-virgin olive oil
2 teaspoons chopped fresh thyme
1 large garlic clove, minced
8 ounces Idiazábal or smoked Gouda cheese, rind trimmed, thinly sliced
6 sourdough rolls (each about 4 inches in diameter), halved horizontally
1 bunch watercress, stems trimmed

Steps:

For aioli:
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:
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.
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.
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.