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Preheat oven to 375°F. In a small bowl, combine butter and oil. With a sharp knife or mandoline, slice potatoes crosswise very thinly.
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In a large baking dish, pour a tablespoon or so of the butter/oil mixture in the bottom and spread it evenly.
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Sprinkle the oil mixture with a few pinches of coarse salt and red pepper flakes, if using; this will allow you to season both the top and underside of the potatoes.
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Arrange your potato slices vertically in the dish.
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Thinly slice onion and slide slivers between potato wedges, distributing them as evenly as possible.
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Brush with remaining oil/butter mixture.
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Generously season your dish with salt.
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Bake 1 1/4 hours, then arrange thyme sprigs on top and bake until potatoes are cooked through with a crisped top, about 35 minutes more.
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If casserole seems to brown too fast, cover it with foil to slow it down.
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Add any garnishes, if using, and serve immediately.