Cook the pasta according to the instructions on the packaging.
Thinly slice the leek and coarsely grate or spiralise the courgette. Mince the garlic and grate the cheese.
Heat the oil in a saucepan, then gently sauté the leek over a medium heat until soft. Add the minced garlic, the chilli flakes and the courgette, then sauté for a minute or so. Season with salt and pepper to your liking.
When the pasta is done, drain it and reserve a small cupful of the cooking water. Add this to the leek and courgette, together with the lemon juice. Turn up the heat and reduce it until creamy (you can add some grated cheese to speed this up).
Toss the pasta with the sauce.
Serve warm with ground pepper, chopped parsley leaves and grated cheese or vegan alternative scattered on the top.