In a small saucepan, dissolve cornstarch in pineapple juice. Add soy sauce, vinegar, garlic and ginger. Simmer until slightly thickened, about 7 minutes. Let cool. Add fish and pineapple to sauce and marinate in the refrigerator for 30 minutes.
Thread fish, pineapple and onion on skewers. Place on a hot grill (or under a broiler) until fish is done to your liking.
Serve with rice. Heat marinade and use as a sauce over fish and rice.
