Heat olive oil in a large soup pot over medium heat.
Add Italian sausage and cook until browned, breaking it apart.
Add chopped onion and cook until softened.
Stir in minced garlic and cook until fragrant.
Add carrots, celery, and Italian seasoning to the pot.
Pour in chicken broth and diced tomatoes.
Bring the soup to a boil, then reduce heat and simmer for 15 minutes.
Add tortellini and cook according to package instructions.
Stir in fresh spinach until wilted.
Season with salt and black pepper to taste.
Remove from heat and garnish with Parmesan cheese before serving.