In a medium skillet, melt 2 Tbsp butter. Add the sliced onions and cook until golden brown and caramelized. Cook them low and slow. Remove from skillet and drain.
In a medium bowl, mix ground beef, salt, pepper and Worcestershire sauce. Mix well and make 4 patties, a little larger than your bread.
Fry the hamburgers in the same skillet you used to cook the onions.
Remove the patties from the skillet and wipe the skillet clean with a paper towel.
Butter the bread. Depending on the size of your skillet, decide how many sandwiches you want to do at a time. Place bread in skillet, butter side down, working quickly, place 1 piece of cheese on bread, 1 hamburger patty, 1/4 of the onions, another slice of cheese and bread, butter side up.
Cook the sandwiches like you would grilled cheese, until nice and golden brown.
**you can also cook some mushroom and place on the sandwich with the onions if desired**