Preheat an outdoor grill for medium-high heat and lightly oil the grate.
Mix egg, garlic, salt, and mushrooms in a bowl. Add ground beef and mix until combined; divide into three patties.
Cook on the preheated grill until the burgers are cooked to your desired degree of doneness, 5 to 7 minutes per side for well done. An instant-read thermometer inserted into the center should read 160 degrees F (70 degrees C).
