Add 1/2 cup of the hot chicken stock to the rice and stir constantly over moderate heat until the liquid is completely absorbed, about 3 minutes. Continue adding the stock, 1/2 cup at a time, stirring constantly until absorbed before adding more. When half of the stock has been added, stir in the prosciutto, peas, rosemary and thyme and continue cooking. The rice is done when it's tender but still slightly firm to the bite. Remove from the heat, stir in the cream and 1/4 cup of the Parmesan and season with salt and pepper. Spread the risotto on a baking sheet and let cool. Cover and refrigerate until chilled, for at least 2 hours or up to 1 day.