Meanwhile, sauté the onion and garlic with the butter in a separate saucepan for 1 minute. Then add the thawed prawns and fry them. When the prawns are the desired orange color, add the cream and crème fraîche and heat. While stirring, add the lemon juice and the grated Parmesan to the sauce and bring to the boil. Season the sauce with vegetable stock, salt and pepper, season to taste and then add the cooked spaghetti to the sauce and stir well for about 1 minute so that the noodles are completely rolled in the sauce.
When the pasta is on the plates, sprinkle some parmesan on top.