Boil the pasta in a large pan of water according to pack instructions. Add the spring greens to the pan 3-4 minutes before the end.
Meanwhile, heat the oil in a frying pan and cook the onion over a medim heat for 5-6 minutes, until browned. Flake the mackerel, discarding any skin and bones, and add to the frying pan. Heat for 2 minutes, stirring often.
Drain the pasta and spring greens, leaving 2 tbsp of cooking water. Return the pasta and greens to the pan, then stir in the mackerel mixture and horseradish sauce. Season and serve immediately.
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