In a 3-qt.saucepan over medium-high heat, saute onion and garlic in oil for 3 minutes.
Add beef; cook until browned, about 6 minutes.
Add the next seven ingredients; bring to a boil.
Reduce heat; cover and simmer for 50 minutes.
Halve each tortilla and cut into 1/4-in.strips.
Gently stir into soup; cover and simmer for 8-10 minutes or until tortillas are softened.