In a large mixing bowl combine the ground beef, chopped onion, minced garlic, rehydrated dried mushrooms and a couple tablespoons of the soaking liquid. Do not overwork the meat, mix until just combined. Divide the beef mixture into four equal portions and form each portion into a 1 inch thick burger patty. Season each side of the burgers with salt and pepper.
Cook the burgers on a grill or grill pan until desired doneness. During the last couple minutes of cooking place 1 slice of cheese on each burger. If using a grill pan tent foil over the pan to help the cheese melt.
Serve the burgers on the buns topping with a large spoonful of the mushroom mixture.
Recipe by Lemon & Mocha at http://www.lemonandmocha.com/2014/01/14/mushroom-overload-cheddar-burgers/