01In a medium bowl, season the steak with salt and pepper.
02In a large skillet, heat oil over medium-high heat.
03Add steak, peppers, and onions. Cook until meat is no longer pink and onions and peppers are tender.
04Place meat and vegetable mixture to the side and clean the skillet.
05Spread 1/2 teaspoon of mayonnaise over one side of each slice of bread. To the non-mayonnaise side of each slice, add 1 slice of cheese, one-fourth of the meat mixture, another slice of cheese, then the final slice of bread, mayo side facing outward.
06In the skillet, heat butter over medium heat.
07Place the sandwich in the skillet (mayo side down) and cook until golden brown on each side, making sure the cheese has melted.