In a medium saucepan over medium heat, melt the butter and saute the green onion until tender. Add the garlic powder, cream of mushroom soup and sour cream. Mix well. Reserve 3/4 of this sauce and set aside. To the remaining 1/4 of the sauce in the saucepan, add the chicken and 1/2 cup of shredded Cheddar cheese. Stir together.
Fill each flour tortilla with the chicken mixture and roll up. Place seam side down in the prepared baking dish.
Return the 3/4 mixture to saucepan and mix with the milk and the other 1/2 cup of cheese.Spoon over the rolled tortillas, pour can of enchilada sauce all over it and then top with cheese. Bake in preheated oven for 30 minutes, or until cheese is bubbly.