Start by making your curry paste. Put the cumin seeds, ground coriander, peppercorns, onion, garlic, ginger, chilli, lemongrass and half of the fresh coriander into a food processor. Add 2 tablespoons of vegetable oil and blitz until smooth.
Heat the remaining vegetable oil over a medium heat in a large heavy bottomed frying pan or wok. Season the chicken legs with salt then place skin side down in the pan. Cook until the skin is golden, then turn over.
Add the curry paste to the pan. Cook for 5 minutes, stirring frequently, until very fragrant.
Add the coconut milk a little at a time, stirring to completely incorporate each time.
Pour in the stock. Add the potatoes (cut into 1 inch cubes, no need to peel) and add enough water to ensure the chicken legs are submerged.
Turn the heat down to low and simmer for 40 minutes.
Turn the heat up and bring to a rolling boil. Add the fish sauce and palm sugar. Stir frequently until the sauce thickens slightly.
Serve with rice and topped with the remaining coriander and a handful of peanuts.