Boil potatoes until tender; about 30 minutes. Drain well. Transfer to a mixing bowl and mash well. Meanwhile, in another saucepan, combine fennel with the broth and cook over medium heat until tender; about 15 minutes. Drain and puree in food processor. Add to potatoes along with the milk and butter. Then transfer to oven-proof casserole; top with Parmesan cheese and bread crumbs. Brown under broiler for a few minutes.