Combine soy sauce, ketchup, vinegar, sugar, broth and cornstarch in small bowl.
Combine 2 tablespoons sauce and the turkey in a bowl. Let marinate at room temperature for 30 minutes.
Heat oil in large nonstick skillet over medium-high heat. Add onion; stir-fry 2 minutes. Add potato, broccoli and pepper; stir-fry 2 minutes. Add 1/2 cup water; cover; cook 6 to 8 minutes, until sweet potato is tender.
Stir remaining sauce. Add with turkey to skillet; cook, stirring, until thickened, 3 minutes.