Rinse the oxtails thoroughly under cold water and pat them dry with paper towels. Season the oxtails generously with salt and pepper on all sides.

Heat a large, heavy-bottomed pot or Dutch oven over medium-high heat. Add a tablespoon of oil to the pot. Once the oil is hot, carefully add the oxtails in batches, making sure not to overcrowd the pot.

Sear the oxtails on all sides until they develop a deep brown crust. Remove and set aside.

In the same pot, add a bit more oil if needed. Add the chopped onions, poblano peppers, and minced garlic. Sauté until the onions are translucent and the mixture becomes fragrant, about 5-7 minutes.

Return the seared oxtails back to the pot with the aromatics. Pour in the fire-roasted diced tomatoes and San Marzano tomato sauce. Add the dried Mexican oregano and stir well to combine.

Pour in the Malbec red wine, stirring gently to incorporate. Bring the mixture to a gentle boil, then reduce the heat to low. Cover the pot and let the oxtails simmer and braise for 3 to 4 hours, or until the meat is tender and falls off the bone. Stir occasionally.

Taste the oxtail stew and adjust the seasoning if needed, adding more salt or pepper according to your preference. Once the oxtails are tender, remove the pot from the heat.

Serve the oxtails hot, either on their own or with a side of rice, mashed potatoes, or crusty bread to soak up the flavorful sauce.

Garnish with fresh herbs like parsley or cilantro if desired, and savor the rich, savory flavors of the oxtail stew!