Preheat the oven to 220°C/450°F/Gas Mark 7. Pat the pork dry with kitchen towel. Stir together the ingredients for the crust. Place the pork in a roasting tin and roast in the oven for 20 minutes.
After 20 minutes, remove the pork from the oven and turn the temperature down to 160°C/320°F/Gas Mark 3. Smear the mustard mixture evenly over the pork. Return to the oven and cook for a further 25 minutes per 500g. Meanwhile heat the butter in a pan and cook the mushrooms with the garlic until tender.
Once cooked deglaze with either lemon juice or white wine and season well. Add the parsley just before serving. Once the pork is cooked, leave to rest for 10 minutes before carving. Serve with the mushrooms.
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