Rinse oxtail under cold water and trim the excess fat. Fill a large pot with enough water, and bring it to a boil, preferably on medium or high heat.
Add chopped onions and oxtail, and simmer until it is tender enough, which should take between 2.5 to 3 hours.
After the oxtail is tender, add peanut butter, and simmer for up to 10 minutes.
Heat the vegetable oil in a frying pan, and sauté chopped onion and minced garlic until they turn golden brown.
Add the eggplant, the bok choy, and string beans. Cook for 5-10 minutes.
Combine the cooked vegetables with the now-tender oxtail in a pot.
Add salt and pepper to taste.
Serve with steamed rice and bagoong alamang, or shrimp paste in the US.