Place the beef oxtails and water in a large pot and bring to a boil. Reduce the heat to medium-low and simmer for at least 2 hours, or until the oxtails are tender. Add additional water as needed during cooking. Skim and discard excess fat from the surface of the broth.

Remove the oxtails from the pot and set aside. Crumble the beef bouillon cubes into the simmering broth and stir until fully dissolved.

Add the bok choy, long beans, and eggplant to the pot. Simmer for 10 to 15 minutes, or until the vegetables are tender. Remove from heat and set aside.

In a separate large pot, heat the olive oil over medium heat. Add the chopped onion and minced garlic and cook, stirring occasionally, until softened and fragrant, approximately 5 minutes.

Stir the achiote powder into the onion mixture until evenly combined. Add the peanut butter and stir continuously until fully melted and smooth.

Gradually add approximately half of the reserved broth to the peanut butter mixture. Simmer for 5 minutes, stirring frequently to prevent sticking.

Return the oxtails to the pot and simmer for an additional 5 minutes. Add the remaining broth and vegetables, stirring gently to combine all components.

Heat through and serve hot.