Trim excess fat from the beef shank, then season with salt and pepper.
Sear the beef in oil over medium-high heat until browned on both sides. Transfer to a plate.
Lower the heat to medium. Sauté the garlic and onion until softened.
Add the beef back in with enough water to cover. Bring to a boil, skim off the scum and fat, then season with salt. Cover and simmer until tender, about 2 hours.
Blanch the sitaw until crisp-tender, then transfer to an ice bath and drain.
Blanch the bok choy until wilted, then transfer to an ice bath and drain.
Fry the eggplant until browned on both sides. Drain on paper towels.
Once the beef is tender, add the annatto.
Add the peanut butter, ground peanuts, and sugar. Stir until smooth, then thicken with slurry. Taste and adjust as needed.
Add the vegetables, then turn off the heat.