Heat a saute pan on medium/high heat. Add the ground beef to the pan. Cook ground beef until browned, for 6-8 minutes.
While the meat is cooking, dice the onion and green pepper. Make sure to remove the core and seeds from the green pepper.
Remove the pan then drain the beef.
Wipe the pan down and return it to the stove. Return the heat to medium/high and add the ground beef back to the pan. Then add the diced green peppers and onions, Worcestershire sauce, salt & pepper, ground mustard, and 1 clove minced garlic.
Cook until the peppers and onions have softened, 4-5 minutes. Turn off the heat.
Preheat the oven to 375 degrees.
Spray a baking dish with olive oil spray. Add the mini-sub buns to the pan standing cut side up.
Stuff the buns with the ground beef mixture evenly.
Sprinkle with cheese.
In a microwave-proof mug or glass measuring cup, add the butter and put it into the microwave to melt for 30 seconds.
Add the remaining minced garlic. Stir.
Slowly pour the melted butter on top of each ground beef cheesesteak that's in the baking dish.
Place the baking dish into the oven and bake the sandwiches for 8 minutes.