Put pork hock in a large pot and fill it with water until level with the pork. Bring to boil over medium high heat until pork is tender. Take pork hock out and reserve the broth.
Heat a deep frying pan or sauce pan over medium heat, add oil and when oil is hot enough, add garlic and onion and cook until fragrant, add annatto powder and ground peanuts and cook for about a minute or two
Add pork broth, about 3 cups, season with fish sauce, bring to a boil then add cooked pork hock. Cover and let simmer for about 10 minutes.
Add peanut butter and stir, adjust consistency to your liking; add string beans and eggplant. Cover again and cook for about 3 minutes, then add bok choy, cover and let simmer for just a minute. Taste and adjust seasoning if necessary.
Serve with rice and sauteed bagoong on the side.