In a medium skillet melt 2 tbsp of butter over medium low heat.
Add the sliced onion and fry until golden brown, stirring everu other minute.
Patty
For the hamburger: place the ground beef, salt, pepper and Worcestershire sauce in a bowl.
Mix well and make 3 patties (each one a little larger than the size of the bread).
In another skillet melt the remaining butter and fry the hamburger patties to your liking.
Remove patties from the skillet, save some of the fat and with a paper towel clean the skillet.
Assembling the sandwich
to make each sandwich: take one slice of bread and place one slice of cheese over it, than one hamburger patty and 1/3 of the onion, another slice of cheese and finish with another slice of bread.
Return the fat you saved from before back to the skillet and grill the sandwiches on both sides until bread is golden brown.