I'm also thinking about layering in some Jerusalem artichokes...can they go in raw?
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No, there is no need to parboil a potato or Jerusalem artichoke before adding them to a gratin. Parboiled potatoes would insure a faster finished dish, while the gratin with the "raw" added ingredient would require a longer cooking time. The down fall to a parboiled ingredient is that they might become "mushy" and fall apart before the "sauce" thickens for the gratin.
FlavorDrizzles is right on. The key is to thin slice, even slice your potatoes and Jerusalem artichoke and give yourself enought time. I cook covered and then uncover for the last 15 to 20 minutes. By the way, love your idea of the J artichoke.
Yes, thin slices are best. I prefer a higher temp oven but most use 325 so it will not over cook. With high temp (400+) you have to watch minute by minute but you will get a great result.