Method
 

1. Pre-heat oven to 180°C: 350°F: Gas 4.
2. Spread the mustard over the skin, season and scatter with a little rosemary (if used).
3. Place in a roasting tin and cook for about 1½ hours, basting occasionally.
   Remove the meat and keep warm.
4. Reserve the cooking juices in the tin, having skimmed the fat.
5. Add the wine and redcurrant jelly to the tin, place over a medium heat and stirring well.
6. Stir in the sour cream and simmer until slightly thickened, add a little cornflour for a thicker sauce.
7. Chop the turnip roughly and boil until soft.
   Drain well and mash.
8. Add the butter, double cream, cummin to the mashed Turnip and season to taste.
9. Carve the lamb, serve with a portion of the turnip puree and roast potatoes and the sauce.