Whisk the sauce ingredients (soy sauce, rice vinegar, cornstarch, and red pepper flakes) in a small bowl. Set aside.
In a large, deep skillet, heat the avocado oil over medium-high heat for about 2 minutes. Add the frozen vegetables, garlic, and ginger. Cook, stirring often, until the veggies are defrosted and tender, about 5 minutes.
Reduce the heat to medium. Give the sauce another stir, then add the turkey meat and sauce to the skillet.
Cook, stirring, until everything is well-coated and the sauce thickens, for about 2 minutes.
Transfer the stir fry to a serving platter. Drizzle it with the sesame oil and scatter the scallions on top. Serve immediately.
Video
Notes
Cook this stir fry minimally. After all, the turkey is already cooked, and you don't want to dry it out. So at the last step, when you add the meat and sauce to the skillet, cook the whole thing no more than the time it takes to heat through, about 2 minutes.
Please don't skip the toasted sesame oil. It greatly elevates the dish.
Leftovers:
Cooked poultry can be kept in the fridge, in an airtight container, for 3-4 days. However, since you're making this stir-fry with leftover turkey, you should take into account how long that meat has already been in the fridge when you decide how long you can store it. As for reheating the leftovers, you can reheat them in the microwave, covered, at 50% power.