2 tablespoons vegetable oil
3 pounds beef oxtail
2 tablespoons all-purpose flour
2 large tomatoes, cubed
1/2 cup red wine
5 cups water, or as needed
1 large yellow onion, cut into 1/2 inch pieces
1 large tomato, chopped
3 carrots, cut into 1/2 inch pieces
1 (14 ounce) can beef broth
2 tablespoons soy sauce
3 tablespoons tomato paste
Heat oil in a Dutch oven or large pot with a lid over medium-high heat. Coat the oxtails in the flour and place in the Dutch oven. Cook until browned on each side. Stir in the 2 diced tomatoes, red wine, and enough water to cover the meat. Bring to a boil, then reduce heat to low. Cook for 3 hours, adding additional water or beef broth if the liquid becomes low.
After 3 hours, skim off some of the fat from the top and heat it in a skillet. Cook and stir the onion until browned, about 8 minutes. Stir in the chopped tomato and carrots. Cook for about 3 minutes. Stir in the beef broth, soy sauce, and tomato paste. Bring to a boil, then stir the onion mixture into the oxtail. Allow to cook for 20 minutes, stirring in more water if the liquid becomes too low.