Cut chicken into large pieces; place in large baking dish or roasting pan. Cut potatoes and celery into quarters, and cut onions into thick wedges with root attached. Add to chicken dish with garlic. Sprinkle with salt, pepper and cayenne, tossing to coat. Nestle California prunes on bottom of pan.
Arrange tomatoes overtop, then drizzle with olive oil. Roast for 20 minutes.
Remove pan from oven; stir in ½ cup water, then gently mix in cilantro. Return to oven and cook until juices run clear when chicken is pierced and potatoes are tender, about 20 minutes.
Drizzle with more olive oil before serving.
Notes
Tip: Arrange your prunes on the bottom of the chicken mixture to prevent them drying out and possibly burning. The juices from the chicken and the tomatoes will drip into the prunes during cooking, creating a magically smooth broth.