1. Thinly slice the onion, bell pepper and mushrooms.
2. Sauté in oil until everything is barely golden and is slightly wilted, season well with salt and pepper and place in a bowl.
3. Form the ground beef into 4 patties and season.
4. Fry the patties in oil for 4 minutes on each side.
5. Now add the sautéed vegetables and, finally, top each patty with a slice of Castello Mature Cheddar.
6. Put the pan in a preheated oven until the cheese has melted. Alternatively, use a pan with high sides and cover with a lid.
7. Spread the warm buns with mayonnaise. Then top with lettuce, the patty and veggies and then the top bun.