Prepare the ingredients: remove the meat from the smoked mackerel (taking care not to leave any bones), chop the washed spinach, finely chop the red onion. Set aside.
Cook the pasta according to the package instructions. Halfway through or towards the end of cooking, reserve a cup of pasta water.
Meanwhile, in a small amount of olive oil, fry the finely chopped onion, then add the spinach. Stir and continue to fry (just until the leaves wilt slightly). Add the mackerel meat, and stir. Finally, add the cream cheese and the reserved pasta water (adjust the amount of water to achieve the desired sauce consistency).
Mix the sauce with the cooked pasta, and season with salt to taste. Optionally, drizzle with a few drops of lemon juice and sprinkle with a tablespoon of grated hard cheese. Enjoy!