A dish that takes time
Rabo de toro is not a quick kitchen exercise. The oxtail pieces are browned hard in olive oil first, then submerged in red wine with carrots, onions, garlic, tomatoes, and bay. Three to four hours at a low simmer, until the collagen breaks down, the meat falls off the bone without any encouragement, and the braising liquid reduces to a sauce thick enough to coat the back of a spoon.
Some cooks add a splash of Pedro Ximénez at the end. The sweet wine dissolves into the sauce and adds a depth that you notice but can't quite identify until someone tells you what it is.