1. Thinly slice the onion, bell pepper and mushrooms. Sauté in oil until barely golden and slightly wilted. Season well with salt and pepper. Place in a bowl.
2. Form the ground beef into 4 patties and season with salt and pepper. Grill the patties for 4 minutes on each side.
3. Top with the sautéed vegetables and a slice of Castello® Extra Mature Cheddar, allowing the cheddar to melt before adding to buns.
4. Spread the warm buns with mayonnaise. Then top with lettuce, the patty and veggies and then the top bun.