Bring a large pot of water to the boil and add the salt. Add the broccoli and cook for about 8 minutes, until the stalks are tender and the florets are bright green.
Lift the broccoli out with tongs or a slotted spoon and transfer it to a blender.
Bring the same water back to the boil and add the pasta. Cook according to the packet instructions (usually around 8 minutes).
After the pasta has cooked for 2-3 minutes, scoop out about 150 ml of the cooking water and pour it into the blender with the broccoli. Add the Parmesan, lemon zest, and half the lemon juice.
Blend until smooth, leaving a small vent for steam to escape. Taste and adjust with more salt or lemon juice if needed.
When the pasta is done, drain it (saving a bit of the cooking water) and transfer to a large bowl.
Add the broccoli sauce and toss to coat, loosening with a splash of the reserved water if needed.
Serve straight away with extra Parmesan on top.
