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1. Preheat oven to 400u00b0F and grease a large (2 quart) gratin or shallow casserole dish.
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2. Cook the bacon in a Dutch oven set over medium heat until done but not too crisp then remove bacon from the pot and drain on a paper towel. Reserve the fat in the pot and keep over heat.
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3. Add 1 tablespoon of the butter to the fat in the pan and melt it, then add the sliced onion and cook until wilted. Transfer the cooked onions to a bowl and set aside.
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4. Add cream, half-and-half, salt and black pepper to the remaining fat in the hot pot and stir to combine.
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5. Add the sweet and russet potato slices into the pot (you're going to separate them after cooked so you might keep them in separate layers) and make sure they are all covered by the liquid. Add more half-and-half if necessary to cover all the potatoes. Increase heat to medium-high heat to bring to a simmer then reduce heat to low and simmer until the potatoes are tender but still firm, about 8 minutes.
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6. Use a slotted spoon to transfer and separate the cooked sweet potato and russet potato slices into two bowls. Let the cream mixture continue to steam over very low heat.
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7. Arrange half of the russet potato slices on the bottom of the greased baking dish. Spread 1/3 of the cooked onions on top of the potatoes and 1/3 of the bacon on top of the onions. Sprinkle 1/3 of the shredded Swiss cheese over everything and then ladle 1 cup of the steaming cream over the cheese.
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8. Make a second layer, using all of the sweet potato slices in place of the russet in the above layering sequence. Add the same layers as above but don't add the cream to this layer.
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9. Top with a final layer using the remaining russet potato slices, all of the same layers as above and this time finish with 1 cup of the cream. Allow the remaining cream to continue to steam very slowly, stirring occasionally, until reduced by half.
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10. Put the gratin dish on a baking sheet, to catch any over spill, and bake for 30 minutes. Remove dish from oven and set it on a rack. Allow to rest for 10 minutes before serving. While the au gratin is resting, stir the remaining 1 tablespoon of butter into the hot cream until blended, adjust seasoning to taste and pour sauce through a wire mesh strainer into a sauce boat.
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Serve this, and any other gratin, in its baking dish-the bubbly, browned crust always brings oohs and ahhs! Serve the reduced sauce on the side.
