Cook sausage in a large pot or Dutch oven over medium-high heat 6-7 minutes, breaking up with a spoon, until browned and cooked through.
Add onion; cook 3-4 minutes until soft. Stir in garlic, Italian seasoning, and optional red pepper flakes; cook 1 minute.
Add tomatoes and broth; bring to a boil.
Stir in pasta; reduce to medium. Simmer 12-14 minutes, stirring occasionally, until pasta is tender.
Stir in spinach and cream; cook 2-3 minutes until spinach wilts.
Remove from heat; stir in Parmesan. Season with salt and pepper; rest 5 minutes to thicken before serving.