Adapted from epicurious.com
1/2 cup mayonnaise
2 teaspoons grainy Dijon mustard
6 teaspoons minced fresh rosemary, divided
1 garlic clove, minced
Nonstick vegetable oil spray
4 large portobello mushrooms, stems trimmed
2 tablespoons olive oil
Pinch of salt, pepper, and Montreal Steak Seasoning
4 brioche buns, toasted
Boston lettuce
Mix mayonnaise, mustard, 2 teaspoons rosemary, and garlic in small bowl.
Spray grill rack with nonstick spray, then prepare barbecue (medium heat). Brush mushrooms all over with olive oil. Sprinkle each with 1 teaspoon rosemary, salt, pepper, and Montreal Steak Seasoning. Grill until tender, about 10 minutes per side.