In a Dutch oven, melt butter over medium heat. Add onion and cook, stirring frequently, 3-5 minutes, until tender and fragrant.
Add the garlic and continue cooking an additional 1-2 minutes.
Sprinkle with flour, salt, and pepper. Continue cooking and stirring another 1-2 minutes, until thickened.
Pour in the broth, undrained tomatoes, tomato paste, and tortellini. Bring to a boil, then reduce heat to Low and allow to simmer 10 minutes, or until the tortellini are cooked through.
Remove from heat then stir in sour cream and spinach. Once the spinach is wilted, adjust the seasoning to taste and serve hot.