In a large pot, heat oil over medium heat until it is very hot. Add mustard seeds and let them sizzle for about 1 minute. Add the garlic, ginger, sliced onion, and cilantro stems, and cook for 5 minutes, stirring frequently. Add the ground spice mix and continue cooking for 6 to 8 minutes until the onions are very soft, stirring often so that the onions cook evenly.

When the onions and peppers are soft and have absorbed the spices, start adding the chicken, a little at a time. Stir the pieces until they're coated with the spices, and then add more chicken. When all the chicken has been stirred in, pour the thin coconut milk over the chicken, add the salt and the potatoes, reduce the heat to medium low, cover and simmer until the chicken is cooked and the potatoes are tender, about 40 minutes.

Remove the cover from the pot and bring the liquid to a boil. Boil for 3 to 5 minutes, until the sauce is thickened. Then add the thick coconut milk and the cilantro leaves. Return liquid to a simmer, season to taste with salt, and serve with rice.

Put a small frying pan over medium heat. When it is hot, add oil. When oil is hot, add coriander, fennel, cumin, cloves, cardamom pods, cinnamon stick, and peppercorns. Cook the spices, stirring, until they begin to brown and release their aromas, about 30 seconds. Immediately transfer spices to a clean coffee grinder or spice grinder, and grind to a powder (you can also grind the spices in a mortar and pestle). Add turmeric and cayenne pepper and mix well.

Soak coconut in 1 cup boiling water for 15 minutes. Transfer mixture to a blender or food processor and process for 1 minute. Line a medium bowl with a double layer of cheesecloth and pour in the coconut-water paste. Gather the edges of the cheesecloth together and squeeze out all of the liquid in the coconut. This will give you about 1 cup of a rich, thick milk. Set aside.