Heat a cast-iron griddle or large heavy skillet over medium-high until it gets very hot. It usually takes about 2 minutes
Lightly brush with vegetable oil
Divide ground beef into 4 equal portions and shape them into a ball
Slice the onion. Make sure your slices are very thin.
Place the ball of meat and bacon on the oiled Cast Iron
Add sliced onions on the patties
Smash the burger with your grill press or spatula
Season with salt and pepper then cook,until outer edges are brown
Flip the slice of bacon and patties. Then season with more salt & pepper.
Place one slice of Mozzarella cheese and one slice of Cheddar cheese on the burger. Cook until the cheese droops
Serve with a soft warm bun, stack 2 patties together, add a lots of mayonnaise, and whatever over toppings you want. Don't forget the bacon and of course... the Cheese Curds!!