Trim fat from pork. Partially freeze pork. Thinly slice across grain into bite-size strips. Set aside.
For sauce, in a small bowl stir together water, sherry, soy sauce, and cornstarch. Set aside.
Pour cooking oil into a wok or large skillet. (Add more oil as necessary during cooking.) Preheat over medium-high heat. Stir-fry ginger and garlic in hot oil for 15 seconds. Add jicama, sweet pepper strips, and green onion; stir-fry 1 to 2 minutes or until crisp-tender. Remove vegetables from the wok.
Add half of the pork to the hot wok. Stir-fry for 2 to 3 minutes or until cooked through. Remove pork from the wok. Repeat with remaining pork. Return all of the pork to wok. Push pork from center of wok.
Stir sauce. Add sauce to the center of the wok. Cook and stir until thickened and bubbly. Add cooked vegetables. Stir all ingredients together to coat with sauce. Add spinach or cabbage. Cook and stir 1 to 2 minutes more or until heated through. Serve immediately with hot cooked rice. Makes 4 servings.