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Preheat oven to 350ºF. Thoroughly rub the inside of a 2-1/2- to 3-quart gratin dish or 3-quart rectangular baking dish with the garlic clove. Generously butter the dish with 1 tablespoon softened butter; set aside.
Using a mandoline or a knife, slice the onion halves very thinly, approximately 1/16 inch thick (1/8 inch with a knife). Warm the oil in a large skillet. Add onions; cook gently, until tender and translucent. Season to taste with salt and pepper and stir in thyme.
In a medium saucepan, heat milk and cream just to a simmer. In a large saucepan, melt the remaining 3 tablespoons butter over medium heat. Whisk in flour. Remove from the heat and gradually whisk in the hot milk and cream. Return to the heat and bring to a boil, whisking or stirring constantly. Boil gently for 3 to 5 minutes or until thickened and the consistency of buttermilk.
Peel russet potatoes and sweet potato. Very thinly slice potatoes on a mandoline to approximately 1/16-inch thick (or slice thinly with a sharp knife). Layer one-third of the potatoes in the prepared baking dish. Season generously with salt and pepper; sprinkle lightly with nutmeg. Top with one-third of the onions and some of the sauce. Repeat layering in thirds, finishing with sauce (cream may not totally cover top).
Bake, uncovered, for 45 minutes. Increase oven temperature to 425ºF. Bake for 10 to 15 minutes more or until bubbly, golden brown and crusty, and potatoes are tender when pierced with a wooden pick. Remove from the oven. Let stand 10 minutes before serving.