Melt butter in a large saucepan or soup pot.Add onion, garlic, carrots, Italian seasoning, salt, and pepper; cook over medium-low heat for 6 minutes, or until vegetables are tender.Whisk in flour; whisk until crumbly.Slowly add broth and continue to vigorously whisk until everything is well combined and there are no clumps.Bring soup to a boil.Add frozen tortellini and spinach.Continue to cook over medium-high heat, stirring occasionally, for 5 to 6 minutes, or until tortellini is tender and heated through.Stir in cream or milk; bring back to a steady boil and cook for 2 to 3 minutes, or until smooth and thickened.Remove from heat and let stand 5 minutes.Taste for salt and pepper, and adjust accordingly.Serve.