1
Marinate the chicken: Use a fork to prick the chicken breasts all over on both sides. Arrange the chicken breasts on a large sheet of plastic wrap, spacing them apart from one another, then cover with a second large sheet of plastic wrap. Using the smooth side of a meat mallet or a rolling pin, beat the chicken until the breasts are an even 1/2- to 3/4-inch thickness. In a small bowl, whisk together the yogurt, 1 tablespoon peanut oil, lime juice, and garlic. Add the pounded chicken, and rub the marinade over the meat. Set the chicken aside while you make the sauce.
2
Make the sauce: In a small bowl, whisk together the coriander, cumin, cardamom, nutmeg, paprika, cayenne, and grated ginger. In a heavy, wide 4-quart pot or saute pan over moderately high heat, melt the butter. Add the onion and saute, stirring occasionally, until light brown and caramelized, about 5 minutes. (Note that because they are sauteed so quickly over moderately high heat, the onions will not caramelize evenly.) Reduce the heat to moderate then stir in the spice and ginger mixture. Add the tomato puree, water, heavy cream, and salt, and bring the sauce to a boil. Reduce the heat to gently simmer the sauce, uncovered, until thickened slightly, about 10 minutes. DO AHEAD: The sauce can be prepared ahead and refrigerated, in an airtight container, up to 3 days.
