Remove the skin from the mackerel - it should peel away easily - flake the flesh into bite size pieces and set aside.
Heat the oil in a large frying pan, add the onion and leek and cook gently for 1-2 minutes. Add the peppers and tomatoes and continue cooking for 4-5 minutes.
Mix the cream cheese, milk and lemon juice together, add to the pan along with the herbs, stir to mix well and until thick and creamy.
Cook the gnocchi according to the instructions on the packet. This normally takes 2-3 minutes. Once cooked, add to the pan along with mackerel and gently fold through the sauce. Tip the mixture into a shallow oven proof dish.
Optional. sprinkle over the breadcrumb and cheese mixture and brown under a hot grill for 5 minutes or until golden brown