Heat 2 tbsp oil in a large, shallow pan. Add the spring onions, garlic and lemon zest, then cook for 2 mins until the onions are tender but still very green. Add the chickpeas, then stir until well coated in the onion mixture. Lightly crush with a potato masher, then add the stock and tomatoes. Simmer for 3-4 mins or until the liquid is absorbed, then set aside to cool slightly.
Meanwhile, heat the remaining oil in a large, non-stick frying pan over a medium heat. Season the mackerel fillets on both sides and fry for 3 mins each side, starting on the skin side. You’ll probably need to cook these in two batches.
Add the basil and a squeeze of lemon juice to the chickpeas, then season to taste. To serve, spoon the warm chickpeas onto serving plates, drizzle with a little extra olive oil and top with the mackerel fillets.