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 liquid is reduced to about 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 the liquid. Stir until smooth and add to the stew.
Strain before you serve
Oxtails produce a lot of fat as they cook so make sure to strain off any excess fat before yous serve.