For the mackerel, preheat the oven to 200C/180C Fan/Gas 6.
Mix the juice of 1 lime and the turmeric. Rub the mixture inside the mackerel, rub any leftover on the outside of the mackerel, set aside.
Heat the oil in a large frying pan and sauté the prawns, garlic, ginger, chilli and curry leaves. When the garlic changes colour add the cumin seeds and stir well until the aroma of the seeds is released. Add the onions and sauté until they are pale and soft.
Blend together the ground cumin and the ground coriander with 100ml/3½fl oz water and and add to the pan. Season the mixture with salt and add the coriander and the remaining lime juice. Remove the mixture from the heat, set aside to cool and then stuff the mackerel with it.
Tie the fish with string in 2 or 3 places to prevent the mixture from falling out or butter a piece of tin foil well and put the fish in the foil folding over on one side and crimping the edges.
Line a baking tray with tin foil and grease with a little oil. Put the fish on the prepared tray and place on the top shelf of the oven. Let the fish stay on one side for 6-7 minutes and then carefully turn over to the other side. Let the skin crisp a bit and colour well, if need be brush some oil or butter on it (if cooking in the foil keep it on each side for between 10-12 minutes).
For the potatoes, bring a large pot of water to the boil, add the potatoes and boil until just done, leave to cool slightly then thinly slice.
Pour enough oil into a large frying pan to coat the bottom of the pan in a thin film. Heat the pan and place the potato slices down on one side and keeping the heat on medium let the potatoes colour lightly. Flip over and while the other side is colouring sprinkle some cumin seeds on the top of the lightly coloured slices.
When the other side is coloured flip the slices over once again and repeat the light sprinkling of cumin seeds on the other side. Cook with the cumin for 15 seconds on each side and then remove. In the same pan add the beef tomatoes and colour them well on each side.
Place the fish on a serving plate and place the potatoes and tomatoes next to it. Serve the seasonal leaves in a separate bowl.