For the yoghurt kadhi, place the yoghurt, chickpea flour, water, salt and turmeric into a bowl and whisk to combine, then pass the mixture through fine sieve.
Place the yoghurt mixture into a pan over medium heat and bring to the boil, whisking constantly. Reduce heat and simmer for 5-8 minutes, or until the sauce turns glossy and begins to coat the back of a wooden spoon. Skim the yoghurt kadhi to remove any scum or impurities that may appear on the surface.
Place the ghee into a small pan and heat to smoking point. Add the red chilli, cumin and curry leaves and as they crackle and cook (almost immediately), pour the oil and spices over the yoghurt kadhi. Set aside.
For the mackerel, preheat the grill to its highest setting.
Place all the spices into a pestle and mortar (or a spice grinder) and grind to a coarse mixture.
Sprinkle the spice mixture over both sides of the mackerel fillets.
Mix the oil and lemon juice together, then drizzle over the mackerel fillets.
Place the mackerel fillets onto a tray and place under the grill, skin side-up, and cook for 8-10 minutes, or until the mackerel is just cooked through.
For the spinach poriyal, heat the oil in a frying pan over a medium heat. Add the mustard seeds and curry leaves and fry for 10-15 seconds (they should crackle and pop.
Add the onions and fry for 5-6 minutes over a low heat, or until translucent.
Add the salt and green chilli and stir-fry for one minute.
Add the spinach leaves and stir-fry for 2-3 minutes, until wilted down.
Add the grated coconut and mix well. Cook for one minute then remove from the heat.
To serve, pour or ladle equal amounts of the yoghurt kadhi into serving bowls and place a mackerel fillet on top of each. Place some of the spinach poriyal over each mackerel fillet and serve immediately.