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Heat 1½ tablespoons of oil in a medium-large saucepan. Add the garlic and rosemary and cook gently for 1 minute, without allowing the garlic to colour. Add the tomatoes and cook briskly for 1 minute, then add the beans and season. Cover the pan and cook for 10 minutes or until the tomatoes have broken down to form a sauce, and the beans are tender but still with a little bite to them.
After about 5 minutes, heat 1 tsp of oil in a large nonstick frying pan. Cut diagonal slashes in the skin of the mackerel fillets, and season on both sides. Add to the hot pan, skin-side down, and cook over a high heat for 2-3 minutes until the skin is crisp. Flip over and cook for a further 1-2 minutes until just cooked through.
Serve the mackerel on a bed of the beans. Drizzle with a little extra oil to serve if you wish.