Ensure you have a large frying pan over a medium-high heat. Heat the oil in a large frying pan. Season the fish fillets with salt and pepper and score the skin a couple of times. Add to the pan skin-side down and fry for 1 to 2 minutes until golden and crisp. Once you have placed the mackerel into the pan add the butter, garlic and parsley to the pan. Turn the fish, tossing the garlic. Carefully turn the fish over, and cook for another 30 seconds, transfer to a plate and keep warm.
Tip in the butter beans, spinach and lemon zest. Add the stock and cook for a minute to wilt the spinach. Season well. Chop the parsley and set aside.
Serve the spinach and butter beans in the base of a shallow bowl and top with the crispy mackerel fillets along with the parsley and a squeeze of lemon juice.