Melt 2 tablespoons unsalted butter in a cast iron skillet over medium-low heat, then add the thinly sliced onions and stir to coat. Cook for 25-30 minutes, stirring occasionally, until deeply golden and caramelized (reduce heat if they brown too fast).

Divide the 80/20 ground beef into 4 balls and season each with Worcestershire sauce, garlic powder, salt, and pepper. Press seasoning into the meat so it stays on during cooking.

Heat the cast iron skillet over high heat until very hot, then smash each beef ball into a thin patty. Cook 2-3 minutes per side, flipping once, until browned and cooked through.

In the same skillet, toast the rye bread slices with butter until golden on the surface. Keep heat medium-high so the bread crisps without burning.

Assemble each sandwich with toasted rye, special sauce, caramelized onions, a smash patty, both cheeses, more onions, and another toast slice. Press lightly so the layers bind.

Return the assembled melts to the skillet and press gently, cooking 1-2 minutes per side until the cheese is fully melted and the bread is crisp. Serve immediately while the cheese stretches.