Cook bacon in a large skillet over medium-high heat for about 3 minutes on each side until nice and crisp. Set aside on a paper towel.
Drain the bacon grease from the pan leaving just enough grease to cook the red onion in (about 1 tbsp). Add the sliced red onion to the skillet and cook on low heat for about 15-20 minutes until soft, stirring occasionally.
While the onions are cooking, form four hamburger patties. Try not to overwork the meat for best results. Rub about a half teaspoon of mustard on each patty. Then, season with salt and pepper and drizzle with a bit of worcestershire sauce.
Heat your grill pan to medium-high heat. If you don't have a grill pan, you can use a cast iron skillet, or just a regular skillet. Add the hamburger patties to the hot grill pan and cook for 5 minutes, undisturbed. Flip patties over and cook for an additional 3 minutes. Place a cheese slice on each patty and turn off heat. Cover and let cheese melt.
Lightly toast the hamburger buns. Spread mayonnaise and ketchup on each bun.
Assemble burgers with cheeseburger patty, caramelized onions and bacon. Serve with french fries or your favorite side dishes!