Bring a large pot of water to a boil. Cook pasta according to package directions. Reserve ¼ cup pasta water, then drain. In a large skillet, cook Italian sausage until lightly browned, breaking it up into pieces with a wooden spoon. Add olive oil, bell pepper and spinach and cook 3-4 minutes or until spinach is wilted. Stir in garlic and Italian seasoning. Add pasta water and pasta to the skillet and season with salt, black pepper and crushed red pepper flakes, stirring to combine. Simmer until liquid has absorbed, stirring constantly. Serve in bowls with more freshly ground black pepper, Parmesan cheese and chopped parsley.