FOR THE PASTRY CRUST: Combine flour and salt, mixing well. Cut in lard until mixture resembles coarse meal. Add water a Tablespoon at a time, while stirring with a fork, just until mixture holds together. Form into ball. Cover with plastic wrap; refrigerate.
FOR THE FILLING: Brown pork in large saucepan over moderate heat, stirring frequently. Add water, onion, sage, and salt; mix well. Cover; simmer over low heat 30 minutes or until meat is tender.
Combine milk and flour; stir to form smooth mixture. Add to pork mixture in saucepan, stirring constantly to prevent lumps from forming. Cook over low heat until thickened and bubbly. Pour 1/2 of pork mixture into 1 1/2 quart lightly greased casserole. Top with apples; sprinkle with sugar. Add remaining pork mixture.
Roll pastry crust. Turn out dough onto lightly floured board; roll to fit casserole dish. Place over meat mixture. Turn under small margin of pastry around edge of casserole; crimp edge. Cut several steam vents in pastry.
Beat egg yolk and milk together; brush crust well with egg wash. Bake at 450°F for 10 minutes. Reduce heat to 350°F and bake 25 minutes more. Serve hot. Makes 4 to 5 servings.