Place meat, onion, and potatoes in a small Dutch oven; add water to cover. Bring to a boil. Reduce heat; cover and simmer until vegetables are tender. Add salt and pepper.
Combine flour and water; stir until smooth. Gradually add to meat mixture; cook over medium heat until thickened and bubbly. Cool and set aside.
Roll half of pastry to 1/8-inch thickness on a lightly floured surface; fit into a deep 9-inch pie plate. Pour meat mixture into pastry shell. Roll out remaining pastry to 1/8-inch thickness; place over filling. Trim edges; seal and flute. Cut slits in top to allow steam to escape. Bake at 350° for 45 minutes or until golden brown.