Cook sausage in a large deep skillet or Dutch oven over medium-high 5-7 minutes, breaking up with a wooden spoon, until browned and cooked through.
Cook onion and pepper 3-4 minutes until softened, then garlic 1 minute more.
Stir in Italian seasoning and optional pepper flakes; add tomatoes and broth and bring to a boil.
Stir in pasta; reduce heat to medium and simmer 12-14 minutes, stirring occasionally, until pasta is tender and sauce thickens.
Season with salt and pepper; sprinkle with mozzarella and cover 2 minutes to melt. Garnish with fresh parsley; serve hot.