Blend the walnuts, bread and herbs together, using a sharp knife make an incision lengthwise and place all the stuffing, roll the meat and tighten with the streaky bacon. Cut the asparagus form the middle, blanch the top part and chop the bottom. Melt the butter and saute the chopped asparagus end, chop the leeks and add. Ones butter starts to brown add the wine and bring to the boil, pour in the cream and simmer until reduced by half. Peel and dice the sweet potato, boil until well softened and mash with garlic butter, season and serve underneath the blanched asparagus.