Mix together the beef mince, onion, mushrooms, garlic, egg white and seasoned salt in a bowl. Form six equal-sized hamburger patties from the mixture. Season burgers on each side with salt and pepper.
Heat a large frypan over medium-high heat. Lie the patties in the frypan and cook until golden brown, about 3 minutes per side. Top each with a slice of Swiss cheese shortly before removing the burgers from the frypan; serve on hamburger buns.