Ingredient:

• 6 hamburger buns, split
• 1/4 cup butter
• 8 ounces sliced mushrooms
• 2 onions, thinly sliced
• salt and freshly ground black pepper
• 2 pounds ground beef
• 1 (1 ounce) packet dry onion soup mix
• 12 slices havarti cheese
• 6 tablespoons mayonnaise