Preheat oven to 375°.
Combine chicken, green onions, 1/4 cup Monterey Jack cheese, Parmesan, chili powder, salt, and pepper in a medium bowl. Place milk and next 3 ingredients (through chiles) in a food processor or blender; process until smooth.
Heat tortillas according to package directions. Pour 1/3 cup tomatillo mixture into bottom of an 11 x 7-inch baking dish coated with cooking spray. Arrange 4 corn tortillas in dish, and top with half of chicken mixture. Repeat layer with remaining tortillas and chicken mixture, ending with tortillas.
Pour remaining 1 1/2 cups tomatillo mixture over tortillas; sprinkle with remaining 1/4 cup Monterey Jack cheese. Bake at 375° for 20 minutes or until bubbly.