Heat oven to 350º F. In a 2-quart baking dish, combine the turkey, sweet potato, zucchini, carrot, scallions, thyme, salt, and a few grinds of pepper and toss well. Pour the gravy on top. Bake in the oven, uncovered, for 25 minutes. Gently roll out 4 biscuits individually and overlap them to make a crust that will cover the baking dish. Bake until the top is golden brown, about 15 more minutes.