In this recipe, portobello mushrooms are layered with grilled onions, blue cheese, and romaine lettuce for a little crunch, then smeared with a special sauce made with yogurt, Dijon, herbs, and garlic. With their meaty texture, portobellos make ideal vegetarian burgers and sandwich fillers. You may be surprised to learn that portobellos (cappellone, or “big hats,” in Italy) are actually oversized cremini mushrooms. Though they are easy to make indoors, we suggest grilling the onions and mushrooms for a nice smoky flavor.
1. Heat a grill pan on medium-high. Lightly brush mushroom caps and onion with oil. Grill mushroom caps and onion for four to five minutes per side or until mushrooms are tender. (Note: Mushrooms will begin to release their juices when tender.) Set aside on a separate plate. Grill buns, cut sides down, for one to two minutes per side or until toasted.
2. Meanwhile, prepare the sauce by combining all sauce ingredients in a small bowl. Spread sauce on bottom half of each bun, dividing evenly. Top with onion, mushroom caps smooth side down, cheese, and lettuce, dividing each evenly. Top with other bun halves.