In a large nonstick skillet, melt butter over medium heat. Add onions and green pepper. Saute until soft (about 12 minutes). While onions and pepper are cooking, slice steak into 1/4 inch slices against the grain. Preheat oven to 400 degrees and put a dark cookie sheet in to get hot. Split the buns in half and butter the insides. When veggies are done, add the steak. Place the buns, butter side down on the hot cookie sheet. Heat steak until warm. Add about a 1/2 cup of queso cheese to hot veggies and meat – you want enough to make it creamy but not soaked. Pull the pan out of the oven when the buns are toasted. Turn your oven to broil. Flip the buns over on the pan, so that the cut/toasted side is up. Fill each bun with 1/4 of the cheesy mixture and place mozzarella on top. Put the sandwiches back in the oven, under the broiled until the mozzarella is melted. Serve hot!