Layer the bean sprouts, tofu, tuna, watercress, tomatoes, and Japanese pickled radish in a 9x13-inch baking dish. Set aside. Place the onion in a bowl.
Heat the garlic in the sesame oil in a skillet over medium heat until garlic is browned and fragrant, about 5 minutes. Remove garlic slices, then pour the hot oil over the onion. Stir in the soy sauce and pour dressing over the salad.