Boil the broccoli in well salted water for 2 minutes and remove with a slotted spoon or spider strainer into a colander. We want the broccoli water left behind to cook pasta in.
Run the broccoli under cold water to stop it cooking. An optional step is to mash up half of it with a potato masher.
Boil your pasta according to package instructions. I like to undercook it by a minute so it can finish cooking in the sauce. Reserve a cup of pasta water before draining.
At the same time the pasta is cooking heat the oil in a large fry pan that will fit all the pasta in at the end. A medium-high heat.
Squeeze the sausage meat out of the casings and fry for around 5 minutes until lightly golden, breaking it up into really small pieces with a wooden spoon. Turn heat down if burning.
Add the garlic and red pepper flakes if using and stir through for 30 seconds until fragrant.
Add broccoli and ½ cup pasta water, bring to a simmer and let it bubble away for 1-2 minutes. It should just be at a gentle simmer so adjust heat accordingly.
Add butter, lemon juice and pepper and stir through to melt.
Add the cooked pasta in and toss well, letting the sauce thicken up slightly for 2 - 3 minutes. There should be some liquid left but it should be reduced down by half.
Arrange everything in the pan into a nice even layer. Lower the heat to low and sprinkle the finely grated parmesan onto the top, letting it gently melt undisturbed for 1-2 mins. I sometimes use a lid to cover the pan to help it along.
Toss vigorously (high sided pans are useful here!) over and over until the sauce has thickened up. It will continue to thicken and coat the pasta off the heat. I find this one gets better a few minutes after as the pasta soaks up the sauce.
Toss the lemon zest through then serve with extra parmesan on top.
The video below will be really helpful!