In a large pot, heat oil over medium-high heat. Sauté garlic and onions until fragrant.
Add oxtail and tripe (if using), cooking until browned on all sides.
Add water and bring to a boil. Skim off any scum that rises to the surface.
Reduce heat and simmer until meat is tender, approximately 1-3 hours depending on cut.
Strain the cooking liquid and reserve the braising liquid. Set meat aside.
In the same pot, sauté additional onion and garlic in oil until softened.
Return the meat to the pot, then add eggplant and string beans. Cook for 5 minutes.
Mix in peanut butter, annatto, and enough braising liquid to create a sauce consistency.
Simmer for 10-15 minutes, stirring occasionally.
If needed, thicken with ground rice until the sauce becomes glossy.
Add bok choy in the last 3-5 minutes of cooking.
Season with salt, pepper, and fish sauce to taste.