Season the oxtail meat with salt and pepper. Place oxtail meat, onion, garlic, and bay leaf in a stockpot along with the 4 cups of water. I use my Instant Pot and cook the meat for 40 minutes until the meat is tender and can be easily separated from the bone. Once the meat is cooked, remove the meat from the bones. On a pressure cooker, the cooking time will be about 50 minutes. If using a regular stock pot, it will take about 1 hour and 45 Minutes, and up to 2 hours.
While the meat is cooking, roast the Poblano peppers by placing them directly over a medium flame. Turn them to have an even roasting, making sure you don’t burn the peppers. Remove them promptly, and place inside a plastic bag for 5 minutes. Meanwhile, roast the tomatoes.
Place the tomatoes on a hot griddle to roast, turning once on every side until it has roasted; you will know it is ready by the formation of a large brown spot. After they are roasted, remove from griddle.
Remove the charred skins from the Poblano peppers by rubbing your finger against their skin (if you can’t remove all the skin, that is ok; the roasted skin adds more flavor to the dish). After discarding the seeds and removing the veins, cut the peppers into strips and set aside.
Chop up the roasted tomatoes and place them in a blender along with the garlic cloves and 1 cup of water. Process until you a have a chunky sauce.
Heat the oil in a large skillet over a medium-high heat. Stir in the onion, sauté for one minute, and then add the Serrano pepper strips. Keep cooking for one more minute. At this time, pour in the blended sauce, 1 cup of the meat’s cooking broth, and the tomato paste. Stir well and cook for 5 more minutes.
Finally, add the meat and Poblano peppers, season the sauce with salt and pepper. Keep cooking for 8 more minutes in a gentle simmer. If the sauce needs more liquid, add more of the beef oxtail cooking broth. To serve, garnish with the chopped cilantro.