Bring the water and vinegar to the boil along with all the other ingredients up to and including the sugar, season with salt. Add the squash, continue to cook for approx. 2 mins., leave to cool in the liquid.
Bring the stock to the boil in a pan. Stir in the polenta, simmer for approx. 30 mins. over a low heat, stirring occasionally to form a thick paste. Spread the polenta approx. 2 cm thick in a square tin (approx. 2 l), leave to cool. Cut the polenta into 8 rectangles. Heat the clarified butter in a non-stick frying pan. Fry the polenta slices in batches for approx. 3 mins. on each side. Top with the mozzarella.
Halve the meat crosswise and then lengthwise. Place the pieces of meat between cling film and pound flat using the base of a pan, season both sides. Arrange the polenta slices on the pieces of meat, roll up tightly.
Heat the clarified butter in a non-stick frying pan. Fry the meat rolls for approx. 5 mins. all over, season with salt. Serve with the squash pickle.