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1
Lift the leeks from the water in which they're soaking so any sand remains in the water.
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2
Drain the leeks in a colander.
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3
Melt the butter and olive oil in a 2-quart saucepan over medium heat and stir in the leeks, and garlic.
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4
Cover and lower the heat.
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5
Cook slowly, stirring occasionally, until the leeks are softened, about 5 minutes.
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6
Add the cream, bring to a simmer, and cook over low heat for 5 minutes.
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7
Stir in the currants or prunes, fresh thyme, salt, and pepper to taste.
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8
Set aside.
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9
Preheat oven to 350 degrees.
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10
Butter a heavy-bottomed 5-quart casserole.
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11
Using a Japanese mandoline or a sharp knife, slice the sweet potatoes 1/4 inch thick.
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12
Do the same with the Idaho potatoes.
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13
Beginning with Idaho potatoes, arrange a layer of potatoes on the bottom of the casserole.
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14
Season lightly with salt and pepper, then spoon 2 tablespoons of leek cream over the potatoes.
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15
Repeat with sweet potatoes: layer, season, and spoon leek cream over them.
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16
Continue in this way, alternating Idaho and sweet potatoes, until all are used. With your hand, or a rubber spatula, press down firmly on the potatoes.
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17
Drizzle the remaining leek cream over the top layer.
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18
Bake 50 to 60 minutes, until the gratin is bubbling around the edges and a knife slides into it easily.
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19
Remove to a warm place and let rest for 15 minutes.
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20
Slice the gratin into wedges or squares and serve.
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21
It may be prepared ahead and reheated in a 300-degree F. oven.
