Heat the olive oil in a large pot or Dutch oven over medium heat. Add the Italian sausage and cook for 5–7 minutes, breaking it into small pieces, until browned; drain excess fat if necessary.

Add the diced onion and cook for about 4–5 minutes, stirring occasionally, until softened. Stir in the minced garlic and cook for 30–60 seconds, just until fragrant.

Pour in the chicken broth, undrained diced tomatoes, and tomato sauce. Add the Italian seasoning, kosher salt, and black pepper, then bring the soup to a gentle boil.

Reduce the heat and simmer the tomato and sausage broth for about 10 minutes. This gives the flavors time to mingle before the quick-cooking tortellini goes into the pot.

Stir in the refrigerated cheese tortellini and cook according to the package directions, usually 3–5 minutes, until tender. Avoid simmering the tortellini longer than needed because it can become too soft or split.

Add the baby spinach and simmer for 1–2 minutes, just until the leaves wilt. Taste the soup and adjust the seasoning with additional kosher salt or pepper as needed.

Ladle the hot soup into bowls and top each serving with grated Parmesan cheese. Serve immediately while the tortellini is tender and the broth is steaming.