3 1/2 pounds oxtails, cut into 2-inch-thick pieces, patted dry and excess fat trimmed
2 1/2 teaspoons kosher salt, divided
1/2 teaspoon black pepper
1/4 cup canola oil, divided
1 large yellow onion, chopped
10 large garlic cloves, peeled and smashed
1 tablespoon chopped drained anchovy fillets (about 6 fillets)
9 cups water
3 tablespoons granulated sugar, divided
1/3 cup plus 1 tablespoon natural peanut butter, well stirred
1/4 cup fish sauce
2 tablespoons annatto oil or vegetable oil
16 ounces fresh wheat noodles, such as Lanzhou or udon noodles, or fresh fettuccine
12 ounces baby bok choy (about 12), trimmed and halved lengthwise
1 small daikon radish, peeled and thinly sliced into half-moons
Sprinkle oxtails with 1 1/2 teaspoons salt and pepper. Heat 2 tablespoons canola oil in a large Dutch oven over medium-high. Working in 2 batches, add oxtails; cook until browned, about 10 minutes per batch. Transfer to a large plate. Discard drippings; rinse Dutch oven, and wipe clean.
Heat remaining 2 tablespoons canola oil in Dutch oven. Add onion; cook, stirring often, until slightly softened, about 4 minutes. Add garlic and anchovies; cook, stirring constantly, until fragrant, about 1 minute. Return oxtails to Dutch oven, and add 9 cups water, 1 tablespoon sugar, and remaining 1 teaspoon salt. Bring to a simmer over high. Reduce heat to very low; cover and simmer until oxtails are tender, about 2 hours.
Pour oxtail mixture through a fine wire-mesh strainer into a large heatproof bowl. Set bowl aside; reserve Dutch oven. Transfer oxtails to a second large bowl; discard remaining solids. Let oxtails cool 15 minutes. Shred and set meat aside; discard bones.
While oxtails cool, skim fat from surface of strained cooking liquid. Pour liquid into Dutch oven, and bring to a boil over high. Add peanut butter, fish sauce, annatto oil, and remaining 2 tablespoons sugar. Transfer mixture to a blender; process mixture until it reaches a creamy consistency, about 20 seconds. (Alternatively, blend mixture in pot using an immersion blender.)
Return blended broth to Dutch oven over medium-high. Add noodles, and cook, stirring occasionally, until noodles are just tender, 5 to 7 minutes. Remove from heat. Stir in bok choy, daikon, and reserved oxtail meat. Cover and let vegetables cook in hot broth until crisp-tender, about 5 minutes. Serve hot.
