01 -
Set your oven to 350°F, then line a baking dish with sturdy foil.
02 -
Melt the butter, sauté shallots until softened. Toss in garlic and herbs, cook briefly. Stir in the flour and let it cook a little. Slowly whisk in the milk to thicken. Add cream, season to taste, and mix in a cup each of gruyere and parmesan. Keep it warm on low heat.
03 -
Peel the sweet potatoes, then slice them thinly (about 1/8 inch). Stack them neatly to make layering easier.
04 -
Start by pouring half of your sauce into the baking dish. Layer the potato slices neatly in a fanned-out style. Pour the rest of the sauce on top. Seal everything tightly with foil.
05 -
Bake for 60 to 75 minutes or until the potatoes are tender. Sprinkle the remaining cheese on top, then broil for 5-8 minutes until golden. Let it cool for about 30 minutes before digging in.