Prepare the dough by combining the butter with flour in a food processor or stand mixer until the mixture is mealy (you can also use your hands). Add the egg and mix thoroughly. Add enough water to form a dough that can be rolled. Put in refrigerator while you make the filling.
Saute the onion and garlic in butter until soft, but not browned. Cool. Mix all filling ingredients with your hands as you would a meatloaf.
Divide the dough into two pieces. Roll one to about 1/8 inch for the bottom. Form filling into an oval loaf on dough and pull up dough to partially cover sides. Roll the other piece of dough to 1/8 thick. Use and egg wash to secure crusts and encase meat loaf in its entirety. Cut a hole in the top to allow steam to escape. Coat entire pastry with egg wash. Left over dough can be cut into decorations and attached to outside of pastry. (I used small Christmas Cookie Cutters to make reindeer, etc.).
Preheat oven to 450 degrees and cook pie about 20 minutes. Turn oven down to 350 degrees and cook 1 hour more or until center registers 150 degrees on a meat thermometer and pastry is browned. Allow to cool.
To make the aspic combine chicken stock with unflavored gelatin and microwave until boiling. Pour through hole in top. Refrigerate 3 hours to serve cold.