Heat EVOO, 2 turns of the pan, over medium-high heat with butter. When butter is melted, add mushrooms and cook to deep brown, about 10-12 minutes. Add salt, pepper, onions and garlic, stir and let soften, about 3-4 minutes more. Take pan off the heat and pour mushroom mixture into a medium-sized bowl and let cool. Chop 8 slices of Swiss cheese and mix it with the cooled mushroom mixture. Set aside.

In a medium bowl, mix ground sirloin with Worcestershire sauce, salt and pepper. Score the meat into 8 sections then roll into 8 balls and form them into very thin patties. Lay 4 of the patties on your work surface, scoop 1/4 of the mushroom cheese mixture into the center of each patty, top with another patty and pinch the sides to seal the mixture inside.

Heat a cast-iron skillet over medium-high heat with one turn of the pan of EVOO. Cook burgers in the pan, about 3-4 minutes per side. Top each burger with a slice of Swiss cheese, cover the pan with a sheet of aluminum foil to help melt the cheese then place on a burger bun. Top with a slice of tomato, some lettuce and onion, and serve.