Season the oxtail chunks with salt and pepper.
Place a deep pot (or Dutch oven) over medium-high heat, then add and heat the olive oil.
Once the oil is ready, start frying the meat.(Note: Do this in batches to not overcrowd the pan.) Brown them on all sides, then remove them from the pan and set them aside.
Using the same pan, sauté the chopped onions.
Once the onions are slightly softer, add the carrots and celery.
Cook for 4 to 5 minutes, stirring frequently.
Add the tomato paste, sweet and hot smoked paprika; then cook for at least 2 minutes.
Add the pieces of browned oxtail back into the pot.
Pour the sherry.
Mix and let at least half of the sherry evaporate. (Note: It generally takes six to seven minutes.) The remaining liquid should be noticeably thicker at this point.
Add the crushed tomatoes and stock.
Bring to a boil, then lower heat and simmer for 2.5 hours or until the meat becomes tender --- taste and adjust seasoning as desired.
Transfer your rabo de toro into plates and serve!