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.
To soften the flavor of the spices, and to make a creamy sauce, the author simmers the chicken for her curry in coconut milk.
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.