1. Heat the olive oil in a large pot or Dutch oven over medium-high heat. Add the Italian sausage, breaking it up with a spoon as it cooks. Stir frequently until the sausage is fully browned and no pink remains, about 5-7 minutes. Drain any excess grease from the pot.

2. Add the chopped yellow onion to the pot with the cooked sausage. Cook, stirring occasionally, until the onion softens and becomes translucent, about 3-5 minutes. Stir in the minced garlic and cook for another minute until fragrant, being careful not to burn it.

3. Pour in the chicken broth and add the undrained diced tomatoes. Stir in the Italian seasoning, salt, and black pepper. Bring the mixture to a gentle simmer, then reduce the heat to low, cover, and let it simmer for 5-7 minutes to allow the flavors to meld.

4. Increase the heat back to medium. Add the refrigerated tortellini to the simmering soup. Cook according to package directions, usually 3-5 minutes, until the tortellini is tender but still firm to the bite. Avoid overcooking, as it can make the tortellini mushy.

5. Remove the pot from the heat. Stir in the heavy cream until fully combined. Then, add the fresh spinach, stirring gently until it wilts into the soup, which should take just a minute or two. The residual heat from the soup will be enough to wilt the spinach perfectly.

6. Ladle the hot soup into bowls. Serve immediately with a generous sprinkle of grated Parmesan cheese on top. A crusty bread on the side makes for a perfect pairing.