Wash the cut oxtail pieces with the lemon juice and cool water. Drain and pat dry.
Season the oxtail with 3/4 of the salt and black pepper, then dust with the flour.
Heat the vegetable oil in a heavy oven-proof pot over medium heat and brown the oxtail on all sides in batches. Set aside.
Leave about 1 tablespoon of oil in the pot. Reduce the heat to medium-low and add the onion, garlic, scallions, thyme, and allspice berries. Cook for 3 minutes. Add the remaining salt.
Clear a space in the center of the pot and add the tomato paste so it touches the heat directly. Cook for 2 minutes.
Pour in the red wine and cook until it evaporates. Return the oxtail to the pot.
Add the chili flakes and beef stock. Bring to a boil. Add the bay leaves and stir in the brown sugar.
Cover and place the pot into a 325°F (165°C) oven for 3 hours, checking every 30 minutes and adding more stock if needed.
Taste and adjust salt. Remove the bay leaves and thyme sprigs. If the gravy is too thin, cook uncovered for 10 minutes.
Once the gravy is rich and the oxtail is tender, top with chopped chives and serve hot. Be mindful of the allspice berries when serving.