In a large pot, bring water to a boil. Add the oxtail chunks and cook for 5-7 minutes to remove impurities. Drain and rinse the oxtail pieces.
Return the oxtail to the pot, cover with fresh water, and simmer for 2-3 hours, or until the meat is tender. You may need to add more water as it evaporates.
In a large skillet, heat the vegetable oil over medium heat. Add the chopped onion and minced garlic, and cook until softened and fragrant, about 5 minutes.
Stir in the annatto powder or achiote paste, cooking for another minute.
Add the cooked oxtail pieces to the skillet, along with the beef broth and 1 cup of water. Simmer for 15 minutes.
Stir in the peanut butter, mixing well to combine and create a creamy sauce. Continue to simmer for an additional 10-15 minutes, until the sauce is thickened.
Add the eggplant slices and string beans to the pot. Cook for 5-7 minutes, or until the vegetables are tender.
Stir in the bok choy or Chinese cabbage and cook for another 2-3 minutes.
Adjust seasoning with salt and black pepper as needed.
Enjoy your meal!