For the poached lamb, place the lamb chop onto a chopping board lined with cling film. Cover with another piece of cling film and flatten with a meat mallet or rolling pin.
In a bowl, mix together the mustard, herbs, salt and freshly ground black pepper. Remove the cling film and spread the mustard mixture over the middle of the lamb.
Roll the lamb up into a sausage shape, wrap tightly in two layers of aluminium foil and tie a knot at each end to seal well.
Heat a pan of water until boiling. Add the lamb parcel, reduce the heat and simmer for ten minutes, or until cooked to your liking. Remove from the pan and leave to rest for five minutes. Remove the foil and slice into three pieces.
For the sweet potatoes, place the sweet potato cubes into a saucepan with the hot vegetable stock, white wine and rosemary and simmer until the sweet potato is tender.
Drain off any excess liquid, add the butter, salt and freshly ground black pepper and mash roughly with a fork.
To serve, spoon the sweet potato onto a serving plate and top with the lamb slices.