Boil salty water and prepare pasta al dente according to the directions on the bag, making sure to reserve at least 1 cup pasta water.
In a large pan over medium-high heat, sauté the diced onion in olive oil with a pinch of salt and red pepper flakes until the onions begin to get translucent and soft.
Add the shaved broccoli and a little more salt, cook for 2 minutes, then add the butter and stir.
Once the broccoli has softened, add the Boursin cheese and about ¾ cup of pasta water.
Stir until the cheese melts into the mixture. If too thick, add more pasta water as needed.
Add the pasta directly from the pot to the sauce, allowing the excess water to help thin the sauce.
Let the noodles finish cooking in the sauce over low heat for 1-2 minutes.
Finish with a squeeze of lemon juice and a sprinkle of red pepper flakes.