Step 1Peel papery skins off tomatillos and rinse well. Place in large roasting pan along with onions and garlic. Roast at 425 degrees until tomatillos have softened and begun to release their juice and onions have browned lightly, about 30 minutes.
Step 2Transfer to blender and blend until smooth, about 1 to 2 minutes. Add salt and pepper to taste. Set aside 1 1/2 cups sauce. Pour remaining sauce into 9-inch skillet.
Step 3Combine chicken, 2 cups cheese, 3/4 cup sour cream and all but 2 tablespoons cilantro. Add reserved sauce.
Step 4Bring sauce in skillet to barely a simmer over low heat. Slide tortillas, 1 at a time, into sauce and cook until tortilla is softened, about 45 seconds. Stack finished tortillas on a plate. Fill each with about 1/2 cup chicken mixture and roll up. Place side by side in 11x7-inch baking dish or place 2 enchiladas in each of 4 (7-inch) individual casseroles. Pour remaining sauce over enchiladas and sprinkle with remaining cheese. Bake at 400 degrees until sauce is bubbling and cheese has melted and browned lightly, about 25 minutes. Garnish with reserved sour cream and cilantro.