Cook the pasta according to the packet instructions, then drain and keep warm.
Meanwhile, thinly slice the leek and cut the asparagus into 3 cm pieces.
Heat the oil in a frying pan and gently sauté the leek for about five minutes, then add the asparagus. Add the lemon juice too. Put a lid on and cook for 3- 5 minutes. If it’s getting too dry, add some of the pasta’s cooking water.
When the vegetables are tender, turn off the heat and stir in the cream and chilli flakes, if used. Add the lemon zest and the chopped herbs, season to taste, then toss with the pasta.
Serve warm.