Heat the olive oil in a large saucepan over medium heat and brown the oxtails and stewing beef all sides. Add water to cover the meat, bring to the boil, reduce heat to a simmer and cook for 30 minutes. Skim off and discard any foam that collects at the top.
Drop in the stock cube, onion, celery, chilli powder, cumin, salt, pepper and corn ears, stir to combine and simmer the soup until the meat is very tender, about 2 hours.
Stir in the carrots and potatoes, simmer for 30 more minutes, then stir in the lentils, rice, mixed vegetables and cabbage. Simmer until the rice, lentils and cabbage are tender, about 30 more minutes.
Serve with a half ear of corn in each bowl, with hot steamed tortillas for dipping in the soup.
If your local supermarket doesn't stock beef oxtail, you can purchase it at your local butchers. You may need to pre-order them.