Cook rotini according to package until it's al dente. Reserve 1 cup of pasta water and set aside.
Add sausage to a skillet and cook over medium heat. Break apart larger pieces as sausage cooks. Once cooked, drain and set aside.
Wipe out the skillet and add a tablespoon of olive oil. Add the garlic and onion, cooking over medium heat just until the garlic is fragrant and the onion has softened, about 2 minutes.
Add broccoli and mushrooms to the pan, cooking 4-5 minutes, or until mushrooms have softened and broccoli becomes a vibrant green color.
Add the sausage back into the skillet and mix.
Stir in the Italian seasoning and add the cooked pasta into the skillet. Mix in the sun-dried tomatoes.
Pour the reserved pasta water into the pan, as well as the juice of 1 lemon and the grated Parmesan cheese.
Season with salt and pepper and serve with crushed red pepper, if desired, and another sprinkle of grated Parmesan cheese on top.