Remove the fins and belly bones from the Norwegian salted mackerel. Slice the fillet diagonally into 8 pieces.

Trim the ends of the green beans and cut them diagonally into 3-4 pieces. Thinly slice the onion and zest the lemon. Squeeze the lemon juice.

Heat a frying pan over medium heat and cook the mackerel skin-side down for 1 minute. Flip and cook for another 2 minutes until done. Remove from the pan.

Boil water in a pot, add salt, lemon zest, and spaghetti. Cook according to package instructions, then drain.

Wipe the pan clean with a paper towel. Heat olive oil over medium heat, add onion and green beans, and sauté for 2 minutes on low-medium heat.

Add heavy cream, milk, salt, pepper, lemon zest, and lemon juice to the pan. Simmer gently, then add the spaghetti and mix well to coat with the sauce.