In a large, deep skillet or Dutch oven over medium-high heat, cook the Italian sausage, breaking it up with a wooden spoon, until browned and cooked through, about 5-7 minutes.
Add the diced onion and bell pepper to the pot and cook for 3-4 minutes until softened. Add garlic and cook for 1 minute more.
Stir in the Italian seasoning and red pepper flakes (if using), then add the crushed tomatoes and chicken broth. Bring to a boil.
Add the pasta to the pot and stir well. Reduce heat to medium and simmer, stirring occasionally, for 12-14 minutes until pasta is tender and sauce has thickened.
Season with salt and pepper to taste. Sprinkle mozzarella cheese over the top, cover with a lid for 2 minutes to melt the cheese, then garnish with fresh parsley and serve hot!