To save time saute the onions and mushrooms up to a day in advance, you can also make this omitting the mushrooms if desired. The cooking time is only estimated depending on the thickness of the sliced potatoes, if you own a mandoline use it for this recipe as the potato slices must be sliced very thinly.
- 2 tablespoons oil
- 2 tablespoons butter
- 3 onions, halved then sliced
- 1 lb white button mushrooms, sliced (can use more mushrooms)
- 3 lbs russet potatoes (peeled and sliced very thin, about 1/8-inch thick)
- 3 cups whipping cream
- 3⁄4 cup blue cheese, crumbled (can use less blue cheese)
- 1 1⁄2 cups mozzarella cheese, grated
- salt and pepper
- Heat oil and butter in a skillet; saute the onions until caramalized (about 10-12 minutes) remove with a slotted spoon to a bowl.
- Add in the sliced mushrooms and saute until they loose there moisture (about 10 minutes) transfer to a bowl.
- Set oven to 375 degrees (oven rack to second-lowest position).
- Butter a 3-4 quart casserole dish.
- 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.
- Transfer one-third of the potatoes using a slotted spoon to the baking dish, spreading them out in an even layer.
- Sprinkle with HALF of the onions, then HALF of the mushrooms; season with salt and pepper.
- The sprinkle ONE-THIRD of both cheeses over the mushrooms.
- Make another layer of potatoes then the sprinkle the REMAINING onions and mushrooms over the potatoes then another third of both cheeses.
- Make one more layer with the remaining potatoes.
- Top with remaining cheeses.
- Pour any leftover whipping cream evenly over the top and cover tightly with foil.
- Make for 45 minutes.
- Increase the oven temperature to 400 degrees.
- Remove the foil and bake until the top is golden brown, and the potatoes are fork-tender (about 12-15 minutes).
- Let stand for 15 minutes before serving (the mixture will thicken upon standing).