Filling: prepare chicken or buy one that's already cooked , shred meat and put in a medium bowl in the fridge.
Sauce: Melt butter In a heavy medium saucepan over low heat. Add flour and cook until light brown, stirring frequently--abt 5 minutes. Whisk in 2 3/4 cups reserved cooking liquid (from chicken...if you bought one, use canned chicken broth). Increase heat to medium and cook until sauce thickens, stirring occasionally. Cool mixture
In another skillet heat oil, add onions, and cook until soft, stirring so they don't stick. Add sauce, sour cream, green onions, and 1 can of chilies. Season with salt and pepper occasionally for abt
Grease two 9/13 baking dishes. Spread 1/2 cup sauce over bottom of each dish. Add one cup of sauce to the chicken and mix well. Mix in 5 cups grated cheese and 1/4 cup chicken broth. Soften tortillas in hot oil or heat a few seconds in buttered dish in microwave. Divide chicken filling among tortillas and roll up. Arrange seam side down in prepared dish. Pour remaining sauce over tortillas. On top of that sauce, I pour a can of green enchilada sauce over the first sauce.
Sprinkle some extra cheese on top. Bake at 400 for about 20 minutes or until bubbly or cover with foil and refrigerate overnight or until ready to bake.
If you don't need to serve 12, make it in two dishes. Freeze one and have one for supper.