Place the oxtail pieces in a large pot and cover with cold water. Bring to a rolling boil over high heat and boil 5 minutes to remove impurities. Drain and rinse the oxtail under warm water, wiping out the pot.

Brown the Oxtail

Heat 1 tablespoon of oil in the same large pot over medium-high heat. Pat the oxtail pieces dry and sear them in batches until browned on all sides, about 6–8 minutes per batch. Remove browned oxtail and set aside.

Simmer the Broth

Return all oxtail to the pot, add 10 cups water, 2 cups beef broth, bay leaves, peppercorns, and thyme sprigs. Bring to a simmer, then reduce heat to low, cover partially, and simmer gently for 2 to 2.5 hours until meat is tender and falling from the bone. Skim any foam occasionally. Remove bones and set meat aside; strain broth into a clean pot and discard solids. Skim excess fat if desired. Season with 1 to 1.5 tsp kosher salt and 1 tsp soy or Worcestershire sauce if using; adjust later.

Aromatics & Mirepoix

While the broth simmers (or after), heat remaining 1 tablespoon oil in a sauté pan over medium heat. Add diced yellow onion, sliced shallots, the white and light-green parts of the chopped green onions, carrots, and celery (if using). Cook, stirring occasionally, until softened and beginning to brown, about 8–10 minutes. Stir in tomato paste and cook 1 minute to deepen flavor. Transfer the cooked aromatics to the strained broth.

Vegetables & Add-ins

Bring the strained broth with aromatics back to a gentle simmer. Add the reserved oxtail meat (shredded or left in chunks) and the potato cubes. Simmer until potatoes are tender, about 15–20 minutes. Taste and adjust seasoning with additional salt and pepper as needed. Add chopped parsley and remove thyme sprigs if used.

Finishing & Serving

Ladle the soup into bowls, scattering sliced green onion tops over each bowl and finishing with a crack of black pepper. Serve hot with crusty bread or rice on the side.