Measure out and form 2 ounce sized meatballs with the ground beef.
Preheat a cast iron skillet over medium high heat.
Spread butter onto the inside half of each of the potato buns.
Place buns, buttered side down, on the skillet and cook until toasted and golden brown, smashing the buns flat as they cook with a large metal spatula.
Flip buns and toast the outside of the buns as well, smashing them flat with the spatula as they cook.
Remove from the skillet, and set the toasted buns aside.
NOTE: The outside of the buns now become the inside of the burgers, and the inside becomes the outside.
After all the buns are toasted, place a meatball in the center of the skillet and use the large spatula to smash the patty as thin as possible.
Season the beef patty with salt and pepper.
Once the first side of the patty is crispy and browned, flip it over and season with additional salt and pepper.
Top the beef patty with slices of red onion and a slice of American cheese.
Cook until the burger patty is crispy and browned on the second side.
Transfer the cooked patty to a toasted bun.
Top the patty with the other bun. The insides facing out and the outsides facing in.
Return to the skillet and toast buns for about 30 seconds to a minute per side, or until the cheese is completely melted.
Serve the burgers with the dipping sauce on the side.