1. Deep-fry shrimp as in Basic Deep-fried Shrimp (but only for 1 to 2 minutes). Then drain. Mince onion and ginger root.
2. In a cup, combine catsup, salt and sugar. In another cup, blend cornstarch and cold water.
3. Heat remaining oil. Add onion and ginger root; stir-fry a few times. Add deep-fried shrimp and stir-fry briefly to reheat.
4. Stir in catsup mixture only to heat through. Then stir in cornstarch paste to thicken, and serve at once.