Heat olive oil in a large pot over medium heat and sauté onion for 4 minutes until translucent.
Add garlic and red pepper flakes, stirring for 60 seconds until fragrant.
Pour in the vegetable broth and add the dried pasta; bring the mixture to a boil.
Reduce heat to a simmer, cover, and cook for 8 minutes, stirring occasionally to prevent sticking.
Place broccoli florets on top of the pasta, replace lid, and steam-cook for an additional 4 minutes until broccoli is bright green and pasta is al dente.
Remove from heat and stir in the heavy cream and Parmesan cheese until the liquid emulsifies into a thick sauce.
Fold in salt, black pepper, and lemon zest, then serve immediately.