In a large pot, boil the oxtail or beef in water until tender. This may take about 1–1.5 hours.
Heat cooking oil in another pot over medium heat.
Add minced garlic and sliced onion, then sauté until fragrant.
Add the cooked beef or oxtail to the pot.
Pour in the beef broth and let it simmer.
Add annatto powder to give the stew its traditional color.
Stir in the peanut butter and mix until the sauce becomes smooth and creamy.
Add eggplant and string beans.
Cook for about 8–10 minutes until the vegetables become tender.
Add fish sauce and salt to adjust the seasoning.
Continue simmering until the sauce thickens slightly.
Serve hot with steamed rice and shrimp paste on the side.