Toast the breadcrumbs in a frying pan with a little vegetable oil, the chilli flakes and garlic until just golden, then set aside. Heat a little more oil in the pan, then add the mackerel fillets, skin-side down, and fry for 3-4 minutes until crisp. Turn over and fry for another 2 minutes.
Meanwhile, bring a large pan of salted water to the boil, then add the spaghetti. Cook according to the pack instructions or until al dente, adding the kale, trimmed of large stems and chopped, for the final 4 minutes of the cooking time. Drain well, reserving 2 tbsp of the cooking water, then return to the pan with the reserved liquid.
Grate in the zest of 1 lemon, then squeeze in its juice. Add 2 tbsp capers and a good drizzle of extra-virgin olive oil. Season well, then flake the mackerel and toss it through the spaghetti, sprinkling over the breadcrumbs to serve.