Do I have to boil the potatoes before making a gratin?

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.

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.

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.