Sauté the chopped onion and zucchini in a large non-stick pan with a drizzle of olive oil. Add the mackerel and rosemary. Cook for 4–5 minutes and season with salt. Crack the eggs into a bowl and whisk with milk, Parmigiano, and a pinch of salt.
Finely chop the onion (if not done yet). Trim the zucchini, cut it lengthwise into quarters remove the seedy center, and dice. Wash and finely chop the rosemary. Flake the mackerel into pieces if necessary.
Boil the pasta in salted water until al dente. Reserve a ladle of the cooking water transfer the pasta to the pan with the mackerel. Add the reserved water and sauté briefly over high heat. Turn off the heat and pour in the egg mixture, stirring quickly so it stays creamy Serve immediately!
