Fill a large saucepan with water, and drop in the oxtail pieces, onion, garlic, salt, and pepper. Bring to a boil, and simmer for 2 hours over medium-low heat, skimming the foam occasionally, until the oxtail meat is very tender and the broth is reduced to 3 cups.
Stir in the eggplant, bok choy, and green beans, and simmer for about 20 minutes, until the vegetables are tender.
Just before serving, place the peanut butter in a small bowl and thin with 1 or 2 tablespoons of broth. Stir until smooth and add to the stew.