Brown the sausage: Heat the olive oil in a large pot or Dutch oven over medium heat. Add the sausage and cook, breaking it up with a spoon, until it is browned and no longer pink. Remove the sausage from the pot and set aside.

Sauté the vegetables: Add the onion to the pot and cook until softened, about 5 minutes. Add the garlic and cook until fragrant, about 30 seconds. Add the mushrooms and cook until softened and browned, about 5 minutes.

Combine ingredients: Return the sausage to the pot. Stir in the crushed tomatoes, diced tomatoes, vegetable broth, pasta, basil, oregano, fennel seeds, and red pepper flakes (if using). Season with salt and pepper to taste.

Simmer: Bring the mixture to a boil, then reduce heat to low, cover, and simmer for 15-20 minutes, or until the pasta is cooked through and the sauce has thickened. Stir occasionally to prevent sticking.

Finish and serve: Stir in the Parmesan cheese and parsley. Serve immediately, garnished with extra Parmesan cheese if desired.