Heat a little oil in a pan over a low-medium heat, then coat the oxtail in flour and add to the pan. After 5 minutes, add the onion and gently brown for 15 minutes, turning regularly.
Add the carrots and parsnips, dissolve the mustard into the stock and add to the pan along with the beer. Add a lid and simmer gently for 2¾ hours, stirring occasionally. The liquid should reduce by half - add a little water if it reduces any more than that.
Serve the oxtail with seasonal vegetables. Mashed potato mixed with horseradish and steamed kale work particularly well.
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