6 corn tortillas (6 inches), halved and cut into 1-inch strips
2 cups sour cream
1 cup shredded cheddar cheese
1 cup shredded part-skim mozzarella cheese
Crumble beef into a large microwave-safe dish. Stir in onion. Microwave, uncovered, on high for 6-7 minutes or until meat is no longer pink, stirring and draining every 2 minutes. Stir in taco sauce and tortillas.
In a greased 2-1/2-qt. microwave-safe dish, layer half of the beef mixture, sour cream and cheeses. Repeat layers. Cover and cook at 70% power for 7-9 minutes or until heated through. Let stand for 2-4 minutes before serving.