In a 4- to 5-quart ovenproof pan over high heat, bring 2 quarts water to a boil. Add taro and cook 5 minutes. Drain and let cool. With a knife, peel taro and cut away any bruised or decayed spots. Cut taro into 1-inch chunks. Rinse pan.
Rinse pork and set, fatty side up, in pan. Add taro, onion, ginger, and broth.
Cover and bake in a 375° oven until meat is very tender when pierced, about 2 1/2 hours. Uncover and stir spinach into juices. Bake until meat is lightly browned, about 30 minutes more. Then broil about 8 inches from heat until meat is richly browned, about 5 minutes longer.
Cut strings from pork, slice meat (it tends to tear apart), and serve with taro mixture and juices. Add salt to taste.