To Prepare Green Chile Sauce: Saute garlic and onion in oil in a heavy saucepan. Blend in flour with a wooden spoon. Add water and green chile and mix well. Add salt. Bring to a boil and simmer, stirring frequently for 5 minutes.
Heat tortillas on a hot griddle and keep warm under a towel or heat in oil, drain on paper towels and keep warm. Mix one cup chile sauce with the chicken. Put 1/4 cup of the mixture on each tortilla and roll it up. Place in a baking dish, cover enchiladas with grated cheese and add onion if desired. Pour remaining sauce over enchiladas and bake at 35 degrees F for about 2 minutes.
Smother with sour cream and return to oven for 1 minutes or until all is hot. Serve immediately