Cook the spaghetti in a large saucepan of boiling salted water, according to the packet instructions, checking for doneness after 10–12 minutes.
Meanwhile, drain the mackerel and remove any excess oil from the slow-roasted tomatoes by dabbing them with kitchen paper. Roughly chop the tomatoes and place in a bowl. Add the mackerel, olive oil and parsley. Season generously with salt and pepper and set aside.
To make the breadcrumbs, heat the oil in a medium–large frying pan over a medium-high heat. Once the oil is shimmering, add the breadcrumbs and gently fry for 3–4 minutes until golden. Remove the pan from the heat and stir in the chilli flakes and lemon zest. Set aside.
Drain the spaghetti, reserving a little of the cooking water, and return the pasta to the pan.
Stir in the mackerel mixture, seasoning to taste with lemon juice and a little more salt and pepper, if necessary. If you want a looser sauce, gradually stir in a little of the reserved pasta water until you have the desired consistency.
Serve the pasta divided between two warm bowls, topped with a generous sprinkling of the breadcrumbs.
