Heat the oil in a large saute pan over medium heat. Add the onions and chiles and lower the heat. Cook, stirring frequently, for about 10 minutes, or until the onions are golden brown and well caramelized
Stir in the ginger and lemongrass and saute for 2 minutes. Add the potatoes and stir to combine. Stir in the black cumin and turmeric. When the spices are well incorporated, stir in the coconut milk and the half-and half. Add salt and remove from the heat
Preheat the oven to 300 degrees F.
Place the chicken in a Dutch oven or another large heavy ovenproof casserole. Pour the coconut mixture over the top of the chicken. Cover and bake for about 1 hour, or until the chicken is very tender
Serve, family style, with basmati rice, a green salad, and warm Indian breads
Note: In India, the chiles would be left in the finished dish, but if you are concerned about them being eaten, remove them before serving.