In a Dutch oven, saute onion in oil until tender. Add garlic; cook 1 minute longer. Stir in the next 13 ingredients; bring to a boil. Reduce heat; cover and simmer for 1-1/2 hours or until beef is tender.
Tear tortillas into bite-size pieces; add to soup. Simmer, uncovered, for 10 minutes; let stand for 5 minutes. Garnish individual servings with cheese, sour cream and onions if desired. Yield: 10 servings (2-1/2 quarts).