In a large heavy-bottomed pot over medium heat, add the ground Italian sausage. Cook, stirring occasionally and breaking the meat into small pieces with a wooden spoon, until browned and no longer pink, about 8–10 minutes. Drain off excess grease if needed.

Add the diced onion to the pot. Cook, stirring occasionally, until the onion is softened and translucent, about 3–5 minutes.

Add the diced tomatoes with their juices, tomato paste, chicken broth, Italian seasoning, kosher salt, and black pepper. Stir well to combine and bring the mixture to a gentle simmer.

Add the cheese tortellini, heavy cream, and fresh spinach. Stir to combine.

Let the soup simmer gently, uncovered, until the tortellini is tender and the spinach is wilted, about 8–10 minutes. Stir occasionally to prevent sticking.

Taste and adjust seasoning if needed. Ladle into bowls and garnish with shaved Parmesan cheese.