Heat the stock. Peel and finely chop the onion annd melt 25g of butter in a frying pan. Fry the chopped onion for 2 minutes, stirring all the time and then add the rice. Mix until it is translucent. Pour the stock onto the rice, a little at a time, stirring constantly, until the rice has absorbed all of the stock. Add the remaining chopped butter, stirring it in. Season and mix well. Take off the heat. Blanch the chive stalks in boiling water for 5 seconds and then drain and refresh with cold water. Lay out the slices of ham on the work surface. Top them with a layer of risotto and place a king prawn at one end of the slice of ham. Roll the ham over on itself, enclosing the prawn and the rice. Knot with a chive stalk and place on a plate. Serve immediately.
