Combine beef, onion, sea salt, and black pepper until ingredients are incorporated.
If using cast iron, place the heat on med-low to medium. If using a nonstick pan, place the heat on med-high.
Now take ¼ of mixture and put some of the butter inside. Form into a thin patty and set on a plate.
Continue with remaining beef.
For every patty add 1 tablespoon ghee/beef tallow to your griddle/pan.
Add patties right in and use a spatula to “smash” the beef down. This helps spread it a bit more to create more surface area that crisps up.
After about 4-5 minutes or until beef is mostly cooked, flip and add 2 slices of cheese per burger.
Cook for about 2-3 more minutes to sear opposite side and melt cheese. You can cover burgers with a lid to help melt cheese faster.
Add buns to the griddle/pan (inside part down) at the end for about 10-20 seconds to warm up and slightly char the bread.
Remove patties and buns from griddle/pan and put some mayonnaise on the bottom buns before putting patties on top. Add whatever toppings you like (I used caramelized onions, sometimes chopped raw onions, jalapeños, ketchup, and mustard).