In a food processor, make a wet masala (spice blend) by pulsing the onion, cilantro (if using), 1 Tbsp of the oil, the lime juice, ginger, paprika, chili powder, garlic, garam masala, salt, cayenne and nutmeg. Pulse until finely minced and fairly smooth.
Meanwhile, drizzle some oil into a heavy skillet, pot or Dutch oven and brown the chicken pieces on all sides, sprinkling with salt and pepper as they cook. Remove the chicken from the pot and set aside on a plate, melt the butter in the pot and add the masala. Cook for about 5 minutes, stirring often. Add the tomato paste and cook, stirring, for another 2-3 minutes. Add the stock and the sugar, bring to a simmer and cook for about 10 minutes.
Return the chicken to the pot, along with any juices that may have accumulated on the plate. Simmer uncovered for 15-20 minutes, stirring occasionally. Add the potatoes, peas and cream or sour cream, reduce the heat to low and cook for 5 more minutes. Serve warm, topped with extra sour cream or yogurt and cilantro, if you like. Serves 4-6.