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1
Melt butter or margarine in a large pot over high heat. Add all mushrooms, and saute; until liquid evaporates, about 10 minutes.
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2
Add garlic, thyme, and rosemary; saute; 1 minute.
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3
Add chicken broth. Simmer until liquid evaporates, stirring often, about 18 minutes.
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4
Season with salt and pepper. Cool.
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5
Position 1 rack in middle of the oven, and another rack in bottom third of oven. Preheat to 375 degrees F (190 degrees C).
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6
Butter a 13x9x2 inch baking dish.
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7
Peel potatoes, and cut into 1/8 inch slices.
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8
Arrange 1/3 of the potatoes in prepared dish, overlapping slightly.
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9
Top potatoes with half of the mushroom mixture.
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10
Sprinkle 1/3 of the cheese over mushrooms.
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11
Repeat layering 1/3 of the potatoes, remaining mushroom mixture, and 1/3 of the cheese.
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12
Arrange remaining potatoes atop cheese.
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13
Whisk half and half, cream, 1 1/4 teaspoon salt, and 1 teaspoon pepper in a large bowl to blend. Pour mixture over potatoes.
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14
Cover loosely with foil.
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15
Place baking dish on middle rack in oven, and a baking sheet on the bottom rack. Bake until potatoes are tender and liquid thickens, about 1 hour and 15 minutes,.
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16
Uncover, using metal spatula, press on potatoes to submerge.
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17
Sprinkle remaining cheese over potatoes.
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18
Bake until cheese melts and gratin is golden at edges, about 15 minutes longer. Let stand 10 minutes.