Mix soy sauce, mustard, garlic powder and pepper in a bowl. Dice onion and add to bowl along with cheese and beef. Mix thoroughly.
Shape beef mixture into 4 hamburger patties. Grill burgers for 6-7 minutes. Flip and grill an additional 3-4 minutes, checking each burger for doneness.
Place burgers on rolls and top with sliced tomatoes and spinach. Serve with raspberries on the side.