Heat the duck fat in a frying pan, put in the lamb and seal on all sides.
Put the chopped mushrooms, shallots and butter in a frying pan and sweat until very soft. Cook the spinach in a pan of boiling water, drain and squeeze dry. Add with all the other stuffing ingredients and mix to a firm paste. Halve the mixture, place each half between 2 sheets of cling film and flatten with a roll