Clean the mackerel, rub them with salt and pour on the juice squeezed from the lemons. Leave them to sit for 30 minutes in a cool area. Heat a little oil in a pan and sauté the chopped leeks and garlic until slightly soft. Season to taste with some salt and black pepper.
Mix with the chopped roasted peppers and lovage. Arrange tomato slices in a greased oven dish, spread the fish on top and cover them with the stewed mixture.
Remove the foil and bake for another 5-10 minutes.