Melt butter in a Dutch oven or soup pot.
Add onions, 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.
Continue to cook over medium-high heat, stirring occasionally, for 5 to 6 minutes, or until tortellini is tender and heated through.
Whisk in cream or milk and add in the spinach; 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.