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1
Heat oven to 400 degrees.
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2
Line a baking sheet with sides with a sheet of parchment.
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3
Place mushrooms gill side up on parchment.
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4
Thinly slice 3 garlic cloves, and scatter over mushrooms.
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5
Place a thyme sprig on each mushroom, drizzle caps with olive oil, salt and pepper, and place in oven until soft and releasing liquid, 10 to 15 minutes.
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6
Remove, and allow to cool.
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7
Keep oven on.
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8
Heat 1 tablespoon butter in a skillet, add onions, and saute over medium-low heat until soft and beginning to color.
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9
Add cream and milk, scald, and remove from heat.
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10
Use remaining butter to grease a 9-inch gratin dish about 2 inches deep.
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11
Rub dish with remaining garlic clove.
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12
Remove and reserve garlic from mushrooms.
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13
Discard thyme.
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14
Slice mushrooms 1/2 inch thick.
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15
Peel and thinly slice potatoes.
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16
Place 1/3 of the potatoes in gratin dish, season with salt, pepper and nutmeg, scatter with half the reserved sliced garlic.
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17
Arrange half the mushroom slices on top.
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18
Repeat layers.
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19
Layer last third of potatoes on top.
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20
Season with salt and pepper.
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21
Pour scalded milk and cream with onions on top.
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22
Strew bread crumbs on top.
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23
Place gratin dish on a baking sheet, and place in oven.
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24
Bake about 1 hour, until potatoes are tender and liquid has thickened.
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25
If top browns too fast, tent with foil.
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26
Allow to stand 10 minutes, then serve.
