In large bowl, combine the oxtail, garlic, tomato, scallion, vinegar, oregano, seasoned salt, curry powder, thyme, pepper, nutmeg and 2 packets Spanish seasoning. Cover with plastic and marinate overnight in the refrigerator.
Heat the oil in large Dutch oven over medium-high heat. Sprinkle the sugar into the pan and cook until starting to caramelize. Add the marinated oxtail pieces and brown on all sides. Add the beef stock, butter beans, some salt and pepper, and the marinade from the bowl. Bring to a boil, then cover and reduce to a simmer. Cook until falling-off-the-bone tender, 2 to 3 hours; towards the end, taste and add more Spanish seasoning as needed. Remove the oxtail to a serving dish.
Whisk the cornstarch together with 1/4 cup water, then whisk into the pot to thicken the gravy. Pour the gravy over the oxtail. Serve with steamed white rice.