Heat cast iron skillet on stove top.
Put a dab of oil on a napkin and rub into skillet.
Mix ground beef into 2 oz. balls, each serving needs 2.
Wrap a pot with aluminum foil to cover the bottom.
Place one ball on hot skillet and smash down with the pot keeping pressure with your hand on the pot (use a towel between your hand and pot) for 1-2 minutes, sprinkle salt and pepper.
Flip burger and smash again with pot and cook for about 30 seconds longer.
Place a slice of cheese on the burger and set aside.
Clean pan between cooking with a paper towel.
Repeat the same for the next one and place the 2nd cooked burger on top of the cheese of the first one. Keep somewhere enclosed to keep warm while you finish cooking.
When all burgers are done, clean pan and toast buns on heated skillet.
Then build your burger.
Lettuce on the bottom bun, then add the burger, then add the tomato, then onion and add ketchup and mustard on your top bun, close and serve.