Boil tomatillos in water with garlic, chiles, salt and pepper until soft, about 15 to 20 minutes.
Pureé cooked sauce in blender to liquefy. While blending, add washed cilantro leaves; set aside.
The sauce yield is about 2 1/2 cups. It will thicken upon standing and you may need to thin with chicken stock.
Simmer chicken in lightly salted water until tender, about 10 to 15 minutes. Cool. Chicken will be slightly under-cooked. Shred cooked chicken and then, just prior to serving, heat in 1 cup chicken stock. This will heat chicken without overcooking.
In medium skillet, heat oil to 300° F. Pass tortillas into hot oil for a few seconds to soften and seal. Remove carefully and set aside between paper towels. Do this just prior to assembly.
Fill softened tortillas with shredded chicken and 1 to 2 tablespoons sauce. Roll up and place seam side down in casserole. Pour green sauce over top and garnish with sour cream and cheese. Place in 375° F oven for 5 to 8 minutes or just long enough to melt cheese.
NOTE: Sauce may be made a day in advance, but the dish is best when chicken is freshly prepared.
Makes: 4