Heat oven to 375°F. Sauté onion in oil in large skillet 3 minutes. Add beef and garlic; sauté 5 minutes. Add tomatoes, chiles, corn, chili powder, salt and cumin. Bring to boil; cook 2 minutes. Remove from heat.
Spread 3 tablespoons enchilada sauce in large deep-dish pie plate (6 cups). Lay 4 tortillas, overlapping, in dish to cover bottom. Spoon on half of meat; sprinkle with 1/2 cup cheese. Layer 4 tortillas over cheese; tuck ends under along edge. Spoon on remaining meat; sprinkle with 1/2 cup cheese. Top with 4 tortillas, then 3/4 cup sauce. Sprinkle with remaining cheese.
Bake in 375°F oven, with foil underneath, 25 minutes. Let stand 5 minutes. Top with scallions. Serve with enchilada sauce and sour cream.