In a large skillet or wok, heat 1 tablespoon of oil over high heat. Add the beef and stir-fry for about 3 minutes. Remove beef and reserve. To the same wok, add the remaining oil and saute the onions, garlic, and ginger. Add the stir-fry vegetable medley, mushrooms, water chestnuts, and red bell peppers. Stir-fry for about 3 minutes and return the beef to the wok. Stirring constantly, add the oyster sauce and crushed red pepper flakes. In a small bowl, whisk the stir-fry seasoning mix with 1/8 cup water until combined. Add the seasoning mixture to the wok and stir for another 3 minutes. Serve with steamed instant white rice.