Place ham in a very large container; cover with cold water. Soak ham overnight.
Scrub ham thoroughly with a stiff brush; place, skin side down, in a deep roasting pan. Add water, vinegar, onion, celery, bay leaves, garlic, and thyme. Bake, uncovered, at 350° for 3 hours, basting every 30 minutes with pan drippings.
Carefully remove ham from pan drippings; discard pan drippings. Cool ham, and remove skin. Combine cinnamon, cloves, allspice, and nutmeg, stirring well. Coat ham with spice mixture, and rub evenly with mustard. Set aside.
Roll pastry to 1/4-inch thickness on a lightly floured surface. Place prepared ham, flat side up, on pastry. Brush edges of pastry with cold water to seal. Fold pastry around ham, trimming excess pastry, if necessary; pinch edges of pastry together to seal.
Place ham, seam side down, in a lightly greased, shallow roasting pan. Prick entire surface of pastry at 1-inch intervals with a fork. Return ham to oven; bake, covered, at 350° for 1 hour. Uncover and bake an additional 40 minutes or until pastry is lightly browned. Allow ham to cool before slicing. Transfer to serving platter, and garnish with Apple Aspic and watercress, if desired.