Insert a meat thermometer into center of meat, roast uncovered in slow 325º oven, until thermometer reaches 140º, rare. Cool, wrap, and refrigerate.
Next day, sauté mushrooms and onion in butter until onion is tender. Combine liverwurst, and mix. Chill until crust is ready.
Mix flour, salt, parsley, and celery seeds, Cut in shortening until mixture is consistency of coarse meal. Sprinkle on cold water, 1 tablespoon at a time, tossing mixture lightly with a fork. Shape dough into a ball and roll out in a rectangle about 1/8” thick, measuring 2” wider than length and circumference of roast.
Pat liverwurst mixture on roast covering top and sides only. Place roast coated side down on pastry rectangle. Fold up pastry to meet along center top and sides. Trim off extra pastry, moistening edges and press together firmly to seal well. Place seam side down in a greased shallow baking pan.
Brush entire surface with beaten egg; prick crust with a fork to vent.
Bake in moderate oven 375º for 1 hour, or until crust is golden brown. Remove from pan and let rest 15 minutes before carving.