Preheat the oven to 375°F, and set aside a pizza pan.
Place a cast iron skillet or large nonstick pan over medium-high heat. Coat the pan with oil, and add the ground beef and diced onions. Stir and cook for about 5 minutes or until the beef is browned and onions are translucent. Strain out the fat that accumulates.
Add in salt and pepper, brown sugar, mustard, ketchup, Worcestershire, vinegar and tomato sauce. Mix thoroughly and reduce the heat.
Simmer and cook the Sloppy Joe mixture for about 6 to 8 minutes until mixture thickens, stirring occasionally. Remove from heat.
Place the bottom half of the buns on a pizza pan (arrange them close to each other), then top them with the cooked sloppy joe mixture. Sprinkle it with cheese. Add the top halves of buns and brush with melted butter.
Bake for about 10 minutes until the cheese is melted. Serve immediately.