To make the fish cakes, half-fill a large saucepan with water and add ½ teaspoon of salt. Bring to the boil and add the potatoes. Reduce the heat and simmer for 15 minutes, or until soft.
Cut the fish fillets in half and put into a large, deep frying pan. Cover with just-boiled water and return to a simmer. Cook for 5 minutes, then drain in a colander.
Put the fish into a large bowl. Drain the potatoes and add to the same bowl. Sprinkle with a teaspoon of salt and roughly mash together with a potato masher.
Heat 1 tablespoon of oil in a frying pan and fry the onion for 4 minutes, or until softened. Add the chillies, ginger and garlic and cook for a few seconds more, stirring. Add another tablespoon of oil with the coriander, cumin and garam masala. Cook for 20–30 seconds, stirring constantly.
Add the spiced onions to the mashed fish and potatoes and mix well. Squeeze the lime juice over and mix in the coriander. Leave to cool for 30 minutes.
To make the spinach sauce, melt the butter in a large saucepan and add the fenugreek. Cook over a medium heat for 30 seconds, stirring. Add the onion and cook for 3–4 minutes, or until soft. Stir in the chilli, garlic and ginger, all the dried spices and a good pinch of salt. Fry for 2 minutes, stirring constantly. Add the frozen spinach and 200ml/7fl oz water and cook gently for 20–30 minutes, stirring occasionally. Stir in the cream and yoghurt and blend with a stick blender until smooth. Adjust the seasoning to taste.
To shape the fish cakes, divide the fish mixture into eight portions and roll each one into a ball. Flatten each into a fish cake shape, roughly 2.5cm/1in thick and 5cm/2in wide.
Put the beaten egg in one bowl and the breadcrumbs in another. One at a time, dip the fish cakes into the beaten egg and then coat in the breadcrumbs, making sure they are fully covered. Put on a baking tray, cover and leave to chill in the fridge for 30 minutes.
To make the masala peas, heat the oil in a medium saucepan and add the cumin. As soon as they start to crackle, add the ginger and chilli and fry gently for a few seconds. Stir in the peas, ½ teaspoon of salt and 250ml/9fl oz of water and cook for 4–5 minutes, stirring until the peas are tender and all the liquid has evaporated. Season with ½ teaspoon of black pepper, add the coriander and mix well. Set aside.
To cook the fish cakes, heat the remaining oil in a large frying pan and gently cook the fish cakes for 4–5 minutes on each side, or until golden-brown, crisp and hot through.
Gently reheat the spinach sauce, stirring constantly and adding a little more cream if needed until the sauce is thick but spoonable.
Squeeze a little lemon juice over the masala peas just before serving them with the fish cakes and spinach sauce.