Pat the oxtail pieces dry with paper towels. Season evenly with 1 tsp kosher salt and 1/2 tsp freshly ground black pepper. Lightly dredge each piece in 2 tbs all-purpose flour, shaking off excess.

Brown oxtails

Heat a heavy-bottomed Dutch oven or large saucepan over medium-high heat. Add 2 tbs vegetable oil and 1 tbs unsalted butter. When shimmering, add oxtail pieces in a single layer without crowding (work in batches if needed). Brown on all sides, about 3–4 minutes per side, until deeply golden. Transfer browned oxtails to a plate.

Sauté aromatics

Reduce heat to medium. If pan looks dry, add a splash (about 1 tsp) of oil. Add the diced onion, carrots, and celery. Sauté until the onion is translucent and vegetables begin to soften, about 5–7 minutes. Add the minced garlic and cook 30–45 seconds until fragrant. Stir in 1 tbs tomato paste and cook 1 minute to remove raw flavor.

Deglaze and build sauce

Pour in 1/2 cup dry red wine (or use 1/2 cup beef stock if not using wine) to deglaze, scraping up browned bits from the bottom of the pot with a wooden spoon. Allow wine to reduce by half, about 2–3 minutes. Return the browned oxtails to the pot. Add 2 cups beef stock, 1 tsp Worcestershire sauce, 2 sprigs fresh thyme, and 1 bay leaf. If using pearl onions, add them now.

Simmer slowly

Bring the liquid to a gentle boil, then reduce heat to low so the surface barely simmers. Cover partially with a lid (leave a small gap) and simmer gently until the oxtail is fall-off-the-bone tender, about 2 to 2½ hours. Check every 30 minutes: skim excess fat/foam from the surface and turn the oxtails gently for even cooking.

Reduce and thicken

When oxtails are tender, carefully remove the meat and bones to a plate. Increase heat to medium-high and simmer the braising liquid uncovered for 8–12 minutes, until it thickens slightly and concentrates in flavor. If you prefer a thicker sauce, whisk 1 tsp cornstarch with 1 tbs cold water and stir into the simmering liquid, cooking 1–2 minutes until glossy. Taste and adjust seasoning with up to 1/4 tsp sea salt and additional pepper as needed.

Finish

Return the oxtails (and any juices released while resting) to the reduced sauce and warm for 1–2 minutes. Remove and discard thyme sprigs and bay leaf before serving.

Finishing & garnish

Sprinkle the stew with 2 tbs chopped fresh parsley and finish with 1/4 tsp freshly ground black pepper. Adjust final seasoning with up to 1/4 tsp sea salt if needed. Serve as a rich side portion alongside mashed potatoes, polenta, or roasted vegetables.