Step 1: Heat a medium cast iron skillet over medium-high heat, add Ground Beef. Using a spatula, break up the beef into small bits. Let cook, undisturbed until the first side is browned, 1-2 minutes.
Step 2: Season the beef with salt, pepper, paprika, garlic powder, and chili powder if desired. Add sliced onion and mushrooms. Reduce the heat to medium, use a spatula to break the beef up into smaller bits and sauté until the onions are soft, mushrooms are tender, and beef is cooked through - about 5 minutes more. Drain off any fat that has accumulated. Turn off the heat, and place 3-4 slices of cheese on top.
Step 3: Place the skillet in the oven using the broil setting until the cheese has melted, 3-5 minutes.
Step 4: Prepare the sauce: combine ketchup, mayonnaise, mustard, vinegar, paprika, garlic powder, relish, and salt in a small mixing bowl. Stir well to combine, and tweak to taste by adding a bit more mustard, salt, or seasoning if needed.
Step 5: Remove the skillet from the oven. Drizzle generously with the sauce. Pile on the toppings, some great options are: iceberg lettuce, pickles, tomatoes, avocado, or jalapenos.