Place oxtail in a 6-8 qt slow cooker. Season with salt and pepper. Arrange the yuca, squash and Cubanelle pepper around the beef in a single layer.
In a separate bowl, whisk together the tomato sauce, 1/2 cup of water, cilantro, garlic, oregano, and Baklouti Olive Oil and pour all over the meat and vegetables. Cover and cook on low until the meat is tender and easy to pull apart (6-8hrs).
Meanwhile, thinly slice the red onion - set aside.
In a small sauce pan, and vinegar, 1/2 cup of water and 1/2 teaspoon of salt to a boil, remove from heat and stir in red onion. Let cool, then transfer to a bowl and refrigerate until ready to serve.
Skim off excess fat from the stew in the slow cooker. Stir the meat in stew and season with salt and pepper (if needed).
Divide into bowls and top with pickled (drained) red onions.