In the bowl of a food processor, combine the yogurt, ginger, turmeric, cumin, paprika, cayenne, garlic, and onion, and process to a smooth puree. Put the puree into a shallow, flat dish and place the chicken breasts into the puree, coating both sides evenly. Allow to marinate, covered and refrigerated, for several hours or overnight.
Preheat oven to 375 degrees.
Season the chicken breasts lightly with salt and pepper and place on a rack in a roasting pan that has been preheated for 10 minutes in the oven. Roast the chicken, turning once, until fully cooked but still moist and not dried out. Cooking time will vary depending on the size and thickness of the chicken breasts.
Alternately, prepare an outdoor grill and cook over indirect heat with the lid on, turning once. Serve the chicken breast with a salad and a grain such as lentils to make a complete meal.