Preheat oven to 400 degrees.
Rub turkey with a small amount of the oil. Season generously with salt, pepper, and thyme. Pour remaining oil into a gratin dish or shallow baking dish large enough to hold turkey. Scatter potatoes, carrots, turnips, and garlic in pan, then put turkey, skin side up, on top of vegetables.
Transfer pan to oven. After 7–10 minutes, pour about ½ cup of the chicken stock over breast. Continue basting every 7–10 minutes, adding the remaining stock by ½ cupfuls. If liquid evaporates, add additional stock or water. Bake in oven until skin is golden brown and an instant meat thermometer registers 168–170 degrees. (A small breast requires about 1 hour.) Remove from pan and let stand 10–15 minutes before cutting into slices. If vegetables are not tender, return pan to oven while turkey rests.
Spoon some roasted vegetables and pan juices onto each plate, and serve.