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1
Melt 2 tablespoons butter (or 1 tablespoon olive oil, 1 tablespoon butter) in heavy large skillet over medium-high heat.
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2
Stir in mushrooms and saute until almost tender, 4 to 6 minutes.
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3
Stir in garlic and saute until mushrooms are tender and golden, about 3 minutes longer.
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4
Transfer mushroom mixture to bowl.
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5
Melt remaining 2 tablespoons butter (or half olive oil, half butter) in same skillet over medium-high heat.
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6
Stir in leeks and thyme and saute until leeks are tender and beginning to brown, 7 to 9 minutes.
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7
Add leek mixture to bowl with mushrooms.
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8
(Can be made 1 day ahead.
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9
Cover and refrigerate.)
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10
Preheat oven to 400F.
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11
Butter 13 x 9 x 2-inch baking dish.
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12
Combine broth, cream, wine, 1 teaspoon salt and 1/2 teaspoon pepper in large bowl, whisk to blend.
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13
Layer 13 of potatoes on bottom of prepared dish.
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14
Top with half of mushroom-leek mixture, then 3/4 cup cheese.
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15
Top with half of remaining potatoes.
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16
Pour half of cream mixture over.
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17
Top with remaining mushroom-leek mixture and 3/4 cup cheese.
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18
Cover with remaining potatoes.
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19
Pour remaining cream mixture over, sprinkle with remaining cheese.
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20
Bake gratin uncovered until potatoes are tender and sauce bubbles thickly, about 1 hour 20 minutes.
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21
Let gratin stand 30 minutes before serving.
