Place 2 tablespoons flour in a large resealable plastic bag. Add beef, a few pieces at a time, and shake to coat. In a nonstick Dutch oven, brown beef in 2 teaspoons butter; remove and keep warm.
In the same pan, saute the onion, celery, carrot and garlic in remaining butter until tender. Add the tomatoes, broth, 1 cup water, thyme, pepper, salt and reserved beef. Bring to a boil. Reduce heat; cover and simmer for 20 minutes. Add tortellini; cook 5-10 minutes longer or until tender.
Combine remaining flour and water until smooth. Stir into the pan. Bring to a boil; cook and stir for 2 minutes or until thickened.