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 quarter 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 180ºC for about 20 minutes.
Smother with sour cream and return to oven for 10 minutes or until all is hot. Serve immediately.