Whisk yoghurt, chickpea flour, water, salt, and turmeric, then pass through a fine sieve.
Boil the mixture while whisking, then simmer for 8 minutes until glossy and thick.
Heat ghee until smoking and fry red chilli, cumin, and curry leaves before adding to the yoghurt.
Preheat the grill to its highest setting.
Grind cloves, fennel, pepper, cinnamon, cumin, and chilli powder into a coarse spice mix.
Apply the spice mix to both sides of the mackerel fillets.
Drizzle the fish with a mixture of oil and lemon juice.
Grill the fillets skin side-up for 10 minutes until cooked through.
Heat oil in a pan and fry mustard seeds and curry leaves until they crackle.
Sauté the chopped onions for 6 minutes until translucent.
Stir in the salt and green chilli for one minute.
Add spinach and stir-fry for 3 minutes until wilted.
Mix in grated coconut and cook for another minute before removing from heat.
Serve fish on the yoghurt kadhi topped with the spinach poriyal.