Halve the lemon and squeeze out. Cut the feta cheese into small cubes, grate the parmesan and stir in the crème fraîche. Halve the avocado, remove the stone and use a spoon to remove the pulp from the skin, add to the feta cheese and pour approx. 1 tablespoon of lemon juice over it. Puree everything with a hand blender. The avocado feta sauce can still be a bit chunky. Season to taste with salt and pepper.
Pat the prawns dry with kitchen paper. Heat a pan and add the olive oil to the hot pan. Add the prawns and the crushed garlic cloves and fry briefly. Season to taste with freshly ground chilli pepper.
Cook the fettuccine al dente and collect some pasta water when draining it. Heat the avocado feta sauce with the collected pasta water, stir in the fettuccine and prawns.