IF I cook these a few hours in advance of a meal, what would be the best way to warm the dish before serving?

Put back on the stove in the same pan, with a splash of stock and a dab of butter (which you should heat together before dropping in the beans); get the pan hot, saute briskly, remove, sprinkle immediately with Parmesan. They don't need to be piping hot to taste great. ;o)