Pat the oxtails thoroughly dry with paper towels. Season all sides with kosher salt and freshly ground black pepper. Heat 1 tablespoon of olive oil in a 6-quart Dutch oven over medium to medium-high heat. Working in batches, sear the oxtails on all sides until deeply browned. Transfer them to a plate and set aside.

Add the chopped onion, carrot, and celery to the same Dutch oven. Cook for several minutes, stirring occasionally, until the onions become translucent and the vegetables begin to soften.

Return the seared oxtails to the pot. Add the whole garlic cloves, bay leaf, thyme, stock, and red wine. Stir to combine. Bring the mixture to a gentle simmer, then reduce the heat to low. Cover and cook for 3 hours, or until the meat becomes fork-tender.

One hour before the stew finishes cooking, preheat the oven to 350°F (175°C). Place the carrots, parsnips, and turnips in a roasting pan. Drizzle with olive oil and season with salt and pepper. Roast for approximately 1 hour, or until the vegetables are lightly browned and fully cooked.

When the oxtails are tender, remove them from the cooking liquid. Skim excess fat from the surface using a spoon, a fat separator, or by chilling the liquid until the fat solidifies and can be removed easily.

Pour the cooking liquid through a fine-mesh strainer into a bowl, pressing the softened vegetables to extract as much flavor as possible. Discard the solids. Return the strained liquid to the Dutch oven and simmer until reduced by half.

Add the oxtails back into the reduced sauce. Incorporate the roasted root vegetables. Warm over low heat for 30 minutes to allow the flavors to meld. Garnish with chopped parsley before serving.