01 -
Set your oven to 350°F and get your baking dish ready by lining it with foil.
02 -
Melt the butter and soften the shallots in it. Toss in minced garlic and herbs, cooking briefly. Sprinkle the flour in, stir, and let it cook for a moment. Slowly whisk in milk until it thickens. Pour in the cream, add seasonings, and toss in 1 cup each of the cheeses. Keep it warm.
03 -
Peel sweet potatoes and cut them into thin, 1/8-inch slices. Keep them stacked neatly for easier layering.
04 -
Pour half the prepared sauce into the dish. Fan the sweet potato slices in even layers, then cover with the rest of the sauce. Wrap tightly with foil on top.
05 -
Bake for 60 to 75 minutes or until the potatoes are soft. Sprinkle the rest of the cheese on top and broil for 5-8 minutes until bubbly and golden. Let it cool for about half an hour before serving.