In a big bowl lightly combine, ground beef and Worcestershire sauce. Do not overmix.
Divide the mixture into 4 equal parts and lightly shape into patties. Do not press or squeeze the patties.
Sprinkle both sides with salt and pepper and make an indentation on the top of the burger.
Heat oil on grill pan or a heavy bottomed pan until smoking, place patties on the pan and cook for 2-3 minutes or until a nice brown crust forms.
Flip burger to the other side and continue to cook until it reached your desired doneness.
Place a piece of Swiss cheese on top in the last minute of cooking to melt cheese.
Remove burgers from pan and set aside covered with a tented foil to rest.
In the same pan, reduce heat a notch and melt 1 tablespoon butter. Add sliced mushroom and cook stirring occasionally for about 5 minutes.
If needed add the remaining butter and sliced onion. Continue to cook until onion caramelizes slightly.
Season with salt and pepper.
Assemble the burgers:
Slather the bottom bun with sauce, top with lettuce then tomato. Place burger on top then add mushroom and onion mix on top of the burger.
Smear the top piece with a little sauce and place on top of burger.