In a 10 inch skillet, over medium heat; saute the onion in the butter until tender. Add the frozen vegetables and cook 3 minutes. Add the cut potatoes and 2 cans of cream of chicken soup. stir to mix well.
Add the cooked chicken to the mixture and thoroughly blend together, stirring often.
Line the bottom of a glass deep dish 10 inch casserole with one of the pie crusts. Cover the bottom and sides with pastry. Scoop the filling from the skillet into the prepared casserole. Spread to make even. Cover with the remaining pie crust. Tuck in around the edges, flute the overlapping edges of pastry to make a design. Cut slits in the top of the crust. Bake 1 hour on 350 until crust is golden brown.