In a bowl, mix together the ingredients for the sauce. Place in the refrigerator until ready to assemble the sandwiches.
For the caramelized onions, melt the 2 Tbsp of butter in a skillet. Add the onion and 1/2 tsp sugar. Cook over medium low heat stirring occasionally for 10-15 minutes until onions have caramelized.
In a bowl, mix the ground beef with the salt, pepper, and worchestershire sauce together. Shape into 3 patties. Cook in a skillet over medium heat until both sides are browned and the patties are cook all the way through.
Butter the outside of all 6 slices of the bread. Assemble the sandwiches by placing 1 slice of bread butter side down. Spread some of the sauce over the bread. then top with a slice of munster and cheddar. Then place the patty on the bread and some of the caramelized onions. Finish by spreading some sauce on the unbuttered side of another slice of bread. Then place butter side up on the sandwich. Grill in the skillet until both sides are browned. Continue to make the other 2 sandwiches the same way. Makes 3 sandwiches.