Add the Worcestershire sauce, salt, pepper, and ground beef to a big bowl. Patty the ground meat into six oval shapes.
Two tablespoons of butter, melted over medium heat in a medium cast-iron pan. Toss in one onion. About 35 minutes into cooking, stir periodically, the onion should be tender and golden brown.
Brown the patties in a large cast-iron pan over medium-high heat, about 2 minutes on each side, or until cooked through. Let the skillet cool down. Take the patties out of the pan and clean the skillet.
Layer 2–3 tablespoons of caramelized onions on top of 1 piece of bread and 1 tablespoon of Secret Sauce. Add 1 slice of cheese and 1 patty on top of that. Finish with 1 more spoonful of Secret Sauce. Finish with a second piece of bread. Proceed with the remaining patties, cheese, caramelized onions, Secret Sauce, and bread.
Sauté the onion in a pan set over medium-high heat. In a pan, melt two tablespoons of butter. Sandwiches need about 3 minutes on each side, or until golden brown and cooked through, so work in batches and turn once. As required, add the remaining butter to the skillet.
