Melt butter in a large pan over a medium heat. Add ham and saute for 5-7 minutes or until just beginning to brown. Add onion and cook for a further minute. Sprinkle flour over the top, stir well, and cook for 1 minute until mixture thickens. Whisk in hot milk, then, once mixture has returned to a simmer, pour in cream and nutmeg. Cook gently for 2 minutes, then season with salt and papper, and fold in swiss cheese.