Pre-heat the flat top grill skillet over medium heat and add a drop or two of oil. Spread the oil with the flat side of your spatula to coat the surface.Place the ground beef in the mixing bowl. Using the salad scoop, form balls of beef, gently releasing them into the hot pan with 2 to 3 inches (5 to 7 cm.) of space surrounding each. (You may only be able to cook 2 to 3 burgers at a time.) Season each beef ball with salt. Grab a golf ball-sized pile of the thinly sliced onion and push it into the center of each ball of beef so it sticks, for the most part. Smash each ball to make a patty. This requires more force than you'd think. Don't worry about smashing the patties too thin—they'll shrink up to the size of your buns as they cook. The onions should fuse nicely with he raw beef. Once they're smashed don't touch again until ready to flip—5 minutes or until red moisture begins to form on the top of the patties. Flip the glorious beef and onion fused patties and slide a slice of American cheese on top of each. Cook for an additional 2 to 2.5 minutes, then transfer to the toasted buns. Serve immediately.