1. Cut steak in half lengthwise (with the grain). Cut steak across grain into 1/8-inch-thick slices. Combine steak, sherry, and pepper in a bowl; toss well.
2. Drain pineapple chunks, reserving pineapple and 1/2 cup juice. Combine juice, soy sauce, and vinegar.
3. Heat oil in a large nonstick skillet or wok over high heat. Add steak; stir-fry 3 minutes or until browned. Remove steak from skillet; drain well.
4. Add onion, ginger, garlic, and 1 cup green onions to skillet; stir-fry 3 minutes or until lightly browned. Add pineapple; stir-fry 1 minute or until lightly browned. Return steak to skillet; add juice mixture. Stir-fry 2 minutes or until thoroughly heated. Serve over rice. Top with sliced green onions.