Cook sliced onions until translucent over low heat to caramelized. Add mushroom and heat until warm. Cool onions and mushrooms.

Mix beef, salt, pepper and Worcestershire sauce well. Make out beef patties to form 4- 6 equal sizes patties.

Make a indention in one patty but don't go all the through. Just a well to hold filling.

Add enough of the onion and mushroom mixture to fill indention in patty. Top the mixture with swiss cheese.

Place another patty to cover bottom patty and press to seal edges of the two patties together. Make sure it is well sealed and not to thin on top or bottom so the goodies inside don't ooze out.

Fry in a medium high heated skillet until it is seared and browned. Flip and cook other side until juices run clear.

Top patties with mozzarella and cover pan to melt the cheese.

Drain on paper towels and toast your favorite bread, bun or roll if desired.

Serve hot but be careful of inside stuffing, it will be hot.

May use your favorite condiments if you like, but I like mine as is. :)