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 350 Deg F For About 20 Minutes. Smother With Sour Cream And Return To Oven For 10 Minutes Or Until All Is Hot. Serve Immediately.