Divide ground beef into 4 patties, approx. 1/2 pound each. Salt & pepper both sides and pan fry over medium heat approx. 4-5 min. per side till done to your taste. Try not to turn them more than once to help keep them juicy. But careful not to burn them. When almost fully cooked, top with cheese.
While burgers are cooking, melt 1/2 stick butter in skillet. Saute fresh mushrooms until browned. Add minced garlic and just heat. Set aside.
Spread Margarine on inside of buns and sprinkle lightly with garlic salt. In small skillet, lightly toast the buns. Assemble burger on bun and top with mushrooms. Enjoy!