Heat the olive oil a large saucepan on high heat. Brown the oxtail pieces all over (top, bottom, sides) for about 10 minutes.
Add the vegetables and brown for a further 5 minutes.
Add the flour, giving everything a good stir, mixing the flour in.
When the flour is fully mixed in, add the red wine, stir then add the stock, mustard and tomato paste.
Let it come to boil, then simmer for about 4 hours, you want to get to the stage where the meat is falling off the bone. Give it more time, 30 minutes more perhaps, if the meat is not quite falling off the bone.
When the meat is done, check for seasoning, add some salt if needed and freshly ground black pepper to taste, because pepper makes everything taste better!
Turn the heat off and leave to rest for 10 minutes.
Stir in the tablespoon of butter and sprinkle the chopped parsley all over, and serve. You could also dish up, then drop a small amount of butter in each bowl, and top with the parsley.. Leave the diner to stir in the butter.