Heat oil in a non-stick pan over medium heat. Sauté the green pepper and onion until soft, (approximately 5-6 minutes). Add in the chopped beef, butter, and steak seasoning; sauté for 3-4 minutes; season with salt and pepper.
Top with cheese and cover with a lid until the cheese has melted. Portion the meat onto the bread. Serve with mustard or your favorite condiments.