Gather all ingredients. Preheat an outdoor grill for medium heat and lightly oil the grate.

Divide ground beef into 6 equal-sized patties; season with meat tenderizer, salt, and pepper. Set aside.

Melt butter in a skillet over medium heat. Add mushrooms and soy sauce; cook and stir until browned. Set aside and keep warm.

Grill patties on the preheated grill for about 2-4 minutes per side. Increase or decrease the cook time according to your desired doneness. For a well done burger, an instant-read thermometer inserted into the center should read 160 degrees F (70 degrees C).

Spoon mushroom mixture evenly on top of patties on the grill; top each one with a slice of Swiss cheese. Cover the grill until cheese melts, about 1 minute.

Remove patties from the grill and serve on hamburger buns.