I don't peel the potatoes because it saves work and the skin adds nice flavor. I cook the potatoes on the stove first, then move things into the oven. This first cooking allows excess water to escape from the potatoes. It also allows you to control the seasoning longer into the cooking process. If the top of the gratin becomes too brown early in the baking stage, cover until there's about 15 minutes left, then remove the cover to get a nicely browned crust.