Combine in medium bowl, Maggi, garlic, pepper, and 1 1/2 teaspoons ginger paste. Preheat large saute pan on medium-high 2-3 minutes. Cut turkey into 1" pieces; add to ginger mixture, turn to coat (wash hands).
Place 1 T. of the oil in pan; swirl to coat. Add turkey (wash hands); cook 3-4 minutes or until internal temperature reaches 165*F. Use a meat thermometer to accurately ensure doneness.
Meanwhile, drain oranges, reserving 2 T. juice; set oranges aside. Place juice and stir-fry vegetables and cook 2-3 minutes or until desired tenderness. Pat water chestnuts dry; stir in during last minute of cook time.
Combine ponzu sauce and cornstarch in small bowl. Pour over vegetables; cook 1-2 minutes, stirring often, or until sauce boils and thickens.
Return turkey to pan, add mandarin oranges; toss gently and serve.