Soak the tamarind pulp in a bowl with the measurement water for 20 minutes. Squeeze the tamarind pulp in your hand to separate the pulp from the seeds and stalks, then strain the pulp through a sieve. Reserve the strained pulpy liquid.
Heat the oil in the base of a tagine or heavy-based saucepan over a medium heat, stir in the onion, chilli, garlic and cumin seeds and cook for 2–3 minutes to let the flavours mingle. Toss in the potatoes and cook for a further 2–3 minutes.
Stir in the powdered spices, tomatoes and sugar and pour in the tamarind water. Bring to the boil, then reduce the heat, cover and cook gently for 15–20 minutes. Season with salt and pepper.
Place the fish steaks in the tagine or saucepan, re-cover and cook the fish gently for 15 minutes. Stir in half the coriander, garnish with the remaining coriander and serve with a salad, if liked.