Season the oxtail with salt and pepper. Heat the oil in a large pot over medium heat. Add the oxtails and brown on all sides, 3-5 minutes per side. Transfer the oxtails to a plate and set aside.

Add the onion to the pot and cook 4-5 minutes to soften. Add the garlic and cook, stirring, 30 seconds, until fragrant.

Deglaze the pot with ¼ cup stock or water and scrape up the browned bits from the bottom. Return the browned oxtail to the pot. Add water/stock to cover the oxtail by 1-2 inches. Bring to a boil, then skim any foam/scum from the surface.

Stir in 1 tablespoon fish sauce and black pepper to taste. Reduce heat and simmer steadily partially covered 2½-3 hours, until the oxtail is very tender.

Turn off heat. Transfer oxtails to a bowl. Reserve the broth and return it to the pot.

In a dry skillet over medium heat, toast the rice flour until lightly golden and nutty-smelling, 3-6 minutes, stirring often. Whisk the toasted rice flour with about 1 cup warm broth from the pot in a bowl until smooth, then whisk it back into the pot.

Whisk in the peanut butter, annatto, and optional brown sugar until smooth and creamy. Keep the stew at a gentle simmer after adding peanut butter.

Add the tender oxtail back to the pot and simmer 10-15 minutes to thicken. If it gets too thick, loosen with more broth.

Bring a large pot of water to a boil. Blanch the eggplant 2-3 minutes until just pliable, blanch the beans 1-2 minutes to slightly soften, and blanch the bok choy 30 seconds until bright green. Drain and set aside.

Ladle kare-kare into bowls or deep plates. Top with blanched vegetables and serve with shrimp paste and crushed peanuts on the side.