41/4-inch-thick slices Vidalia or other sweet onion
116 oz pkg sliced baby bella mushrooms, optional
Cooking spray
2teaspoonssherry vinegar
41 1/2-ounce whole-wheat hamburger buns, toasted
Instructions
Preheat grill to medium-high heat.
Combine 1/2 cup blue cheese, mayonnaise, 1 teaspoon thyme, and Tabasco sauce in a small bowl; stir well and set aside.
In a medium bowl, combine ground beef, soy sauce, Worcestershire sauce, steak sauce, and 1 teaspoon pepper. Mix well. Divide beef into 4 equal portions, shaping each portion into a 1/2-inch-thick patty. Sprinkle beef evenly with 1 teaspoon black pepper and salt.
Brush oil evenly over both sides of onion slices; sprinkle lightly with salt and pepper. Place the patties and onions on grill rack coated with cooking spray; cover and grill for 3 to 4 minutes on each side. Meanwhile, in a grill pan sprayed with cooking spray, quickly saute mushrooms.
Set patties aside; keep warm. Place onion slices and sauteed mushrooms in a zip-top plastic bag with vinegar and thyme; seal. Let stand 5 minutes.
Spread cut sides of buns evenly with mayonnaise mixture. Arrange lettuce and tomato on bottom half of each bun; top with 1 patty, 1/4 of onion and mushroom mixture, and bun top.