Drain juice from peaches and combine with the 2 tablespoons flour, and a pinch of salt , which have been sifted together. Cook over direct heat , until mixture boils and thickens. Remove from heat and add butter and lemon juice. Combine with peaches and pour baking dish. Roll out the chilled pastry on a floured board to fit on top of a casserole; gash in several places and lay over peaches. Bake in a hot oven (450° F.) for 20 to 25 minutes or until crust is browned.