For the potatoes
Peel and dice the potatoes. Peel the onions and cut into wedges. Halve, core and dice the yellow pepper. Halve the lemon grass lengthwise. Wash the orange leaves.
Heat the oil in a wok and fry the onion wedges, then take out of the wok. Fry the potatoes gently in the wok, then add the yellow pepper, lemon grass and orange leaves. Add the sugar, fish sauce and curry paste. Then add the coconut milk and stock granules. Bring to the boil briefly.
Wash the coriander and puree finely in a liquidiser or food processor. Season the sauce to taste with the coriander. Serve the potato curry in bowls with the onions.
For the curry
Put the chillies into a small bowl and cover with lukewarm water. Leave to stand for 30 minutes. Meanwhile peel and finely dice the shallots. Clean and finely chop the coriander root. Cut the lemon grass into thin rings, using only the yellow part. Take the chillies out of the water and pat dry. Put all these ingredients into a mortar and grind to a smooth paste. Transfer the paste to a bowl and stir in the cumin and turmeric.