Bring a large pot of water to a a boil. Rinse the ox tail with cold water and cut into smaller pieces at the joints. Blanch the ox tail for about 2 minutes in the boiling water. Remove and drain off the water. Clean the pot and get ready to do the cooking
Preheat the pot and then add in oil. Add in onions and stir-fry until soft and fragrant, about 2 minutes. Add in the tomatoes and ox tail and stir-fry for another 1 minute. Add in the sauce you've mixed in the bowl earlier and enough water to cover the ox tail. Bring to a boil and then lower the heat to let it gently simmer away for the next 2 hours or until the ox tail is really tender and the liquid is lightly reduced. During cooking, make sure you stir every now and then to prevent burning. You can turn the heat up a little bit if the liquid is not reduced by end of cooking. Have a taste and season with some salt if needed
Turn the heat back up to let the remaining liquid boil a little bit and then stir in the corn starch mixture to let the sauce thickened. Serve immediately