Combine pork, 1 teaspoon cornstarch, white pepper, and 1/4 teaspoon salt in a medium bowl; set aside.
Combine remaining 2 teaspoons cornstarch and 1 tablespoon water in a small bowl; set aside.
Combine ketchup and next 3 ingredients (through soy sauce) in a small bowl; set aside.
Heat oil in a large nonstick skillet over medium-high heat. Add pork mixture and garlic; sauté 2 minutes. Remove pork from pan; set aside. Add cabbage; sauté 2 minutes. Add ketchup mixture; sauté 2 minutes. Stir in pork; sauté 1 minute. Stir in cornstarch mixture; cook 30 seconds or until thick. Remove from heat; stir in onions.