Heat oven to 350°F. Spray 13x9-inch (3-quart) baking dish with nonstick cooking spray. Place 2 cups of the tortilla chips in bottom of sprayed baking dish. Layer chicken over chips.
In medium bowl, combine onions, tomatillos, green chiles, cumin and garlic; mix well. Spread onion mixture over chicken. Drop teaspoons of sour cream over onion mixture. Sprinkle with cheese. Cover with foil.
Bake at 350°F. for 30 minutes. Uncover; sprinkle with remaining 1 cup tortilla chips. Bake uncovered for an additional 20 to 25 minutes or until thoroughly heated. Serve with salsa.