Cook the pasta according to the package directions, reserving 1/2 cup of the pasta cooking water (put a ladle in your colander so you don't forget the pasta water).

Meanwhile, brown the sausage in a large skillet over medium-high heat, breaking up the meat as it cooks. Transfer the sausage to a plate.

Melt the butter in the same pan over medium heat. Add the onion and cook for 10 minutes, until soft and golden brown. Return the pork to the pan.

Add the garlic, Italian seasoning, 1/2 teaspoon salt, and 1/4 teaspoon black pepper and stir to coat. Cook for 1 minute, until the herbs are fragrant. Add the wine and bring to a simmer. Simmer for 1 minute, until the liquid is almost completely evaporated, scraping up any browned bits from the bottom of the pan. Add the chicken broth and bring to a simmer.

Fold in the spinach. Add the pasta and reserved pasta water and cook for 2 to 3 minutes, until the spinach wilts and the sauce thickens. Fold in the parmesan cheese and season to taste with salt and black pepper.

Top with crushed red pepper flakes (if using) and serve with extra parmesan cheese on the side.