Preheat oven to 400°F. Line a sheet pan with foil. Heat butter in a 10-inch nonstick skillet over medium heat. Add mushrooms and onion. Season with salt and pepper to taste. Sauté until vegetables are tender.
Meanwhile, combine ground beef with 1/2 teaspoon salt, Cajun Seasoning, and Worcestershire sauce.
Divide mixture into 4 portions. Shape meat into bowls on the prepared sheet pan.
Fill meat bowls with sauteed vegetables. Top with cheese. Loosely cover with foil, and bake for about 20 minutes or until cooked through: 165°F.
Meanwhile make burger sauce by mixing together mayonnaise, ketchup, mustard, and relish
Serve burger bowls topped with shredded lettuce, diced tomatoes, and burger sauce.
