Trim rind, if any, from ham. Shave ham fat covering to about 1/4 inch. Make X-shaped cuts with a small paring knife, 2 inches deep and 1 inch apart, staggering rows, all over fat side.
Cook kale in boiling salted water to cover, following directions on package; drain; cool; squeeze out excess water with hands. Combine kale, spinach, onion, watercress, celery tops, salt and pepper in a medium-size bowl.
Press greens mixture into ham pockets, packing down well. Place ham, fat side up, in a large shallow baking pan. Bake in a slow oven (325 degrees) for 2 hours.
Stir honey with vinegar and dry mustard; brush part over ham. Continue baking and brushing with remaining honey mixture, 30 minutes, or until top is richly glazed. Remove ham from pan and let stand about 20 minutes for easier carving.
Carve ham carefully, holding slices to keep filling intact. Garnish platter with buttery glazed carrots, cooked buttered squash and watercress, if you wish.
