In a large pot over medium heat, cook the Italian sausage until browned. Drain excess grease if needed.
Add the diced onion and garlic, cooking until softened.
Pour in the chicken broth and diced tomatoes, then add Italian seasoning, red pepper flakes, salt, and pepper.
Bring to a simmer, then add the tortellini and cook according to package instructions.
Stir in the heavy cream and fresh spinach, cooking until the spinach is wilted.
Serve hot, garnished with grated Parmesan cheese.