Gratin Dauphinois with Sweet Potato and Arugula-Ginger Salsa

The origins of this dish can be traced back to the former province of Dauphiné in southern France. Usually made with new potatoes only, in this recipe we replace half of them with sweet potatoes and parboil the potato slices to halve the oven time. Make the salsa when the gratin comes out of the oven so the arugula remains fresh and crunchy. Allowing the gratin to rest for a few minutes will make it firmer and easier to cut into neat slices.