Butter each burger bun with softened butter, set aside.
Heat your griddle to very high heat.
Coat a small area with cooking spray and add onions, moving around until soft and opaque, with a little char, about 2-3 minutes. Move to side or off the griddle entirely.
Divide ground beef into 8 2-ounce balls.
Working in batches (unless you have a large griddle), place 4 balls onto the hot surface and immediately smash with a strong, flat spatula. Do not move, just let them sizzle. Cook for about 30-45 seconds. Season from 12 inches above with Kosher salt and freshly ground pepper.
Using the spatula, scrape up the patties and flip. Top with cheese sauteed onions and one slice of cheese, cooking for another 30 seconds.
Remove to a plate.
Immediately, place buttered burger buns down on the hot griddle to char for about 1 minute or until browned. I actually do this to both sides to get the butter effect all over, but that is up to you.
Remove and assemble burgers, 2 patties per burger bun.
Add any desired condiments.
If you've tried this recipe, come back and let us know how it was in the comments or ratings.
Do not use lean ground beef. You need a good amount of fat to prevent sticking and get the browned and crispy crust.