How to Make It
Prepare the peas: Heat a Dutch oven over high, and add canola oil. Sprinkle oxtails evenly with ½ tablespoon of the salt and 1 teaspoon of the black pepper, and place in Dutch oven. Cook until browned on all sides, about 4 minutes per side. Remove oxtails from Dutch oven. Pour off drippings, leaving browned bits in Dutch oven, and add stock. Bring stock to a boil over high, using a wooden spoon to scrape browned bits from bottom of Dutch oven. Return oxtails to Dutch oven; reduce heat to medium-low, cover, and simmer until tender, 1½ to 2 hours. Using a slotted spoon, transfer oxtails to a plate, and let cool slightly.
Add peas, onions, carrots, celery, jalapeño, garlic, thyme sprigs, and bay leaves to stock mixture; reduce heat to low, and cook, partially covered, until peas are soft, about 1 hour. Stir in remaining 1 tablespoon salt and 1 teaspoon black pepper. While peas cook, pick oxtail meat from bones, and set aside.
Prepare the rice: While the peas finish cooking, preheat oven to 300°F. Bring water, salt, and cayenne pepper to a boil in a medium saucepan over medium-high. Reduce heat to medium; add rice, and stir. Simmer gently, uncovered, stirring occasionally, until rice is al dente, about 15 minutes.
Drain rice in a fine wire-mesh strainer, and rinse under cold water. Spread rice out on a rimmed baking sheet. Dry rice in preheated oven 10 minutes, stirring occasionally. Scatter butter evenly over rice, and continue to dry, stirring every few minutes, until all excess moisture has evaporated and grains are dry and separate, about 5 more minutes.
Add oxtail meat and cider vinegar to peas. Using a slotted spoon, transfer peas to a large serving bowl. Add rice, and carefully toss together. (Pour enough oxtail gravy over the top to moisten the peas without making the dish too soupy, about 1 cup.) Garnish with scallions.