How to Make It
In a food processor, puree the yogurt with the onion, garlic, ginger, lemon juice, turmeric, cumin, coriander, cinnamon and cayenne. Place the turkey breasts on a large rimmed baking sheet. Pour the yogurt marinade over the turkey and rub it over both sides and under the skin. Cover and refrigerate for at least 6 hours and up to 24 hours. Remove the turkey from the refrigerator 1 hour before roasting.
Preheat the oven to 475°. Transfer the turkey breasts (with any marinade that sticks to them) to a large roasting pan, skin side up. Season with salt and pepper and drizzle the butter over the skin. Roast for 20 minutes. Reduce the oven temperature to 375° and roast for 50 minutes longer, or until an instant-read thermometer inserted in the thickest part of the breast registers 165°. Transfer the turkey to a cutting board and let rest for 10 minutes. Carve and serve.