Heat a skillet over medium heat.
Add ground beef, season with salt and pepper, and cook until browned.
Add chopped onions and bell peppers; sauté until soft.
Stir in the cheese until melted and well combined.
Toast the sandwich bread in a separate pan until golden brown.
Spread mayonnaise on the toasted bread.
Spoon the beef and cheese mixture onto one slice of bread.
Add fresh lettuce and tomato slices on top.
Cover with the second slice of bread.
Press down gently and slice in half before serving.