Bring a large saucepan of salted water to the boil, add the spaghetti and stir well. Simmer for 12-15 minutes until cooked.
While the pasta cooks, prepare the sauce. Whisk together the horseradish sauce, yoghurt, eggs and parmesan. Season with black pepper.
Heat a large pan until medium hot, add the oil and stir fry the courgette for 2 minutes until it has started to soften.
Add the smoked mackerel, mix well then cook for another minute. Stir in the spinach and cook for a couple of minutes until wilted down.
Turn the heat off (to prevent curdling), add the horseradish and yoghurt mixture into the pan. Stir really quickly until mixed throughout the courgettes and mackerel.
Drain the pasta and tip straight into the pan, with about 100ml of the cooking water, then stir until totally covered in the sauce.