Heat skillet over medium heat. Add 1 tablespoon butter and sliced onions with a pinch of salt. Cook 8-10 minutes, stirring occasionally, until golden and caramelized. Set aside.
Divide ground beef into 4 equal portions (about 4 oz each). Roll loosely into balls. Season outside with salt and pepper.
Heat cast iron skillet over medium-high heat for 3-4 minutes until very hot.
Place one beef ball in pan. Immediately smash down hard with spatula (use a second pan on top for pressure) until about ΒΌ-inch thick. Hold pressure for 5 seconds. Cook undisturbed for 2-3 minutes.
Flip patty, immediately top with cheese slice. Cook 1-2 minutes until cheese melts. Remove and repeat with remaining patties.
Wipe pan clean. Butter one side of each bread slice. Toast butter-side-down in pan over medium heat until golden, about 2 minutes.
On toasted side of bread, spread Thousand Island if using. Stack 2 patties and onions. Top with second bread slice, butter-side out. Return to pan and toast 2 minutes per side until golden.
Slice in half and serve immediately with pickles.
