Place shrimp under cool running water till partially thawed; set aside.
Drain pineapple, reserving juice.
Combine reserved pineapple juice and cornstarch; stir in soy sauce.
Place onions, oil, and gingerroot in 1 1/2-quart casserole.
Cook, uncovered, at HIGH for 3 minutes till tender, stirring once.
Stir in soy mixture and water chestnuts.
Cook, uncovered, at HIGH for 4 minutes till thickened and bubbly, stirring after each minute.
Stir in shrimp and pineapple.
Cook, covered, at HIGH for 2 minutes till heated through, stirring once.
Serve with hot cooked rice or chow mein noodles, if desired.