Season chicken with salt and pepper. In large skillet, heat 2 Tbsp butter and oil. Add chicken and cook over medium high heat until light brown on both sides. Remove from pan and set aside. Cook onion in skillet until soft, but not brown. Cut all but 3 chilies into strips. Add strips to onion and cook over medium heat 5 minutes. Remove from heat and set aside. Place 3 whole chilies, 1/4 cup milk and 1/2 tsp salt in blender or food processor. Process until smooth. Add sour cream and blend a few seconds more. Set aside. Melt remaining 2 Tbsp butter in small saucepan. Blend in flour. Add 3/4 cup milk and cook, stirring until smooth and thickened. Remove from heat. Stir in sour cream mixture. Arrange half the chicken breasts in a casserole. Top with half the onion chilies mixture. Spread with half the sauce. Repeat layers. Bake, covered, @ 375 degrees for 25 minutes. Remove from oven. Sprinkle with cheese. Bake 3 to 4 minutes. Serve over beds of fluffy Arroz Blanco. Makes 6 servings.
ARROZ BLANCO: Melt 2 Tbsp butter or margarine in 2 to 3 quart saucepan. Add 1 cup uncooked rice and cook over moderate heat until golden, stirring often. Add 1/2 cup chopped onions and 2 cloves minced garlic; cook until onions are soft but not brown. Add 2 cups chicken broth and 1 tsp cumin seed. Bring to a boil; stir once or twice. Reduce heat, cover, and simmer 15 minutes or until rice is tender and liquid is adsorbed. Salt to taste.