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1
Heat oil and butter in a skillet; saute the onions until caramalized (about 10-12 minutes) remove with a slotted spoon to a bowl.
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2
Add in the sliced mushrooms and saute until they loose there moisture (about 10 minutes) transfer to a bowl.
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3
Set oven to 375 degrees (oven rack to second-lowest position).
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4
Butter a 3-4 quart casserole dish.
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5
In a 2-quart saucepan combine the potatoes with the whipping cream (the potatoes on top do not have to be covered completely) bring to a boil; remove from heat and cool to warm (about 30 minutes) leaving the potatoes in the cream.
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6
Transfer one-third of the potatoes using a slotted spoon to the baking dish, spreading them out in an even layer.
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7
Sprinkle with HALF of the onions, then HALF of the mushrooms; season with salt and pepper.
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8
The sprinkle ONE-THIRD of both cheeses over the mushrooms.
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9
Make another layer of potatoes then the sprinkle the REMAINING onions and mushrooms over the potatoes then another third of both cheeses.
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10
Make one more layer with the remaining potatoes.
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11
Top with remaining cheeses.
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12
Pour any leftover whipping cream evenly over the top and cover tightly with foil.
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13
Make for 45 minutes.
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14
Increase the oven temperature to 400 degrees.
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15
Remove the foil and bake until the top is golden brown, and the potatoes are fork-tender (about 12-15 minutes).
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16
Let stand for 15 minutes before serving (the mixture will thicken upon standing).
