Preheat the oven to 425°F (220°C). Lightly oil a large baking pan that can accommodate the oxtails in a single layer.
Rub the oxtail sections with olive oil and season with salt and black pepper. Place the diced onion and roughly chopped garlic in the prepared pan, then top with the oxtails.
Roast in the preheated oven for 25 minutes. Flip the oxtails, continuing to roast until they're well browned, about 20-35 minutes more.
Add the tomato sauce, chicken broth, sherry vinegar, fresh thyme, oregano, rosemary, kosher salt, and red chili flakes to the baking pan. Stir well, then transfer the mixture to a large pot. Heat over medium-high until the sauce begins to simmer.
Cover the pot, reduce heat, and let it simmer on low for 4 to 5 hours, until the oxtails are tender and easily detach from the bones.
Before serving, skim off any excess fat from the surface.
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