In a large bowl, combine the beef, two pineapple slices (chopped finely), soy sauce, onions, ginger, pepper and garlic. Divide the beef mixture into four equal portions, shaping each into a ½-inch thick patty.
Preheat grill. Place burgers on the grill rack; grill four minutes on each side or until desired degree of doneness and grill the remaining pineapple slices. Serve on toasted buns topped with two pineapple slices on each burger. Serve with desired condiments.
Note: For added flavor try topping with a splash of steak sauce, light mayo, spinach leaves and havarti cheese.