Cook the spaghetti al dente according to the instructions on the packet.
At the same time, heat the oil or butter. Peel and finely dice the onions. Peel off the garlic cloves and chop (or press, however you like). Fry both in it, then add the cream cheese to the pan and heat it up. If the sauce is not liquid enough for you, you can add some cream (although the crabs also give off liquid). Add the crabs or prawns to the pan and season to taste.