Bring water to boil in a large stock pot. Cook potatoes until tender, then drain. Mash with butter and milk, adding salt and pepper to season.
Preheat oven to 350 . Place one prepared pastry shell into an 8" pie plate and set aside.
Heat oil in a large frying pan. Add onion, celery, and carrots, sautéing until soft. Stir in allspice. Add ground meat and cook until browned. Stir in mashed potatoes and taste, adding salt and pepper as needed. Add mixture to pie shell and top with the second crust. Cut several holes into the top of the pie, then, using a fork, seal the bottom and top crusts together. Bake for 60 minutes. Let cool before serving.