Add ground beef to a skillet and heat over medium high heat, stirring frequently until beef is browned. Drain grease and return beef to pan.
Add Worcestershire sauce, garlic powder and season with salt and pepper. Add onion, peppers and butter and continue to heat over medium heat for 5 minutes.
Add beef stock or broth and bring mixture to a boil. Reduce heat to simmer and continue to cook for about 6-8 minutes or until liquid is reduced to half.
Turn off heat and add cheese on top. Cover and let stand for 2-3 minutes or until cheese is melted. Roughly stir the cheese into the beef and serve inside buns.