Cook the pasta according to the instructions on the packaging.
Thinly slice the leek and cabbage. 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 and stir for a minute or so. Season with salt and pepper to your liking.
When the pasta is done, drain it and reserve a cupful of the cooking water. Add this to the leek together with the cabbage. Cover and sauté for 5 minutes until the cabbage has softened.
Add the cream, grated cheese, lemon zest and juice. Stir until you get a thick sauce.
Add the drained pasta and walnuts, then toss well.
Serve warm with ground pepper, chilli flakes and extra grated cheese scattered on top.