Salt and pepper the oxtail, then dry roast it in a 350°F (176°C) oven for 1 hour. Turn every 15 minutes.
Pour rendered fat into a pan and cook mirepoix, lemongrass and garlic until tender.
Add sachet, oxtail and red wine.
Add dashi and bring to a boil. Once it boils, put a lid on it and place it in a 350°F (176°C) oven for 21⁄2 hours until tender. When it’s done, let it cool to room temperature.
Pick the meat from the bones OR transfer it to a nonreactive (glass or plastic) pan and let it sit overnight in the refrigerator.
The next day, gently warm the stock and pick the meat from the bones. Discard bones. Set aside the meat.
Reduce 4 cups (0.95 l) stock to 1 cup (237 ml). Shred the meat and add it to reduced stock. Add soy sauce, fish sauce and sugar. Stir and cook until mixture resembles marmalade.
Set aside or cool and warm later. Oxtail marmalade will last in refrigerator 1 week.
Soak the bone marrow in water with a pinch of salt until ready to use. Preheat the oven to 400°F (204°C). Season the bone marrow with salt and pepper, then place it cut side down on a roasting pan and bake 8 minutes. Remove bone marrow from the pan and place sliced bread into rendered fat. Place the bone marrow back on top of the bread (2 pieces of bread per piece of marrow). Cook 6 minutes or until the internal temperature of the bone marrow reaches 140°F (60°C).
When the bone marrow is done, put it on a tray. Season it with sea salt, black pepper, espelette chili flakes and chives. Flip bread onto a plate, toasted side up. Top the bread with oxtail marmalade. Pair it with fresh radishes and pickled citrus.