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1
Preheat the oven to 450 degrees F.
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2
Place the potatoes on a cutting board.
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3
With the tip of a small, sharp knife, score a lid shape on the top of each potato about a third of the way down, cutting in about 1/8 inch all the way around.
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4
Spread a 1/4-inch-thick bed of rock salt on a baking tray.
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5
Place the potatoes on the bed of rock salt.
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6
Bake the potatoes until tender enough to be pierced easily with a wooden skewer, about 45 minutes.
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7
Remove the baking sheet from the oven and leave them for a few minutes to cool slightly.
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8
Leave the oven on.
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9
Heat a saute pan over high heat.
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10
Add the olive oil and, as soon as it swirls easily, add the butter.
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11
When the butter foams, add the ham and mushrooms and saute until they are hot, 2 to 3 minutes.
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12
Transfer the ham and mushrooms to a small mixing bowl.
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13
Using a small, sharp knife, and protecting your hand from the heat with a folded kitchen towel, carefully cut off the lids from the still-hot baked potatoes, following the scoring marks.
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14
With a sharp-edged tablespoon, scoop the flesh from the potatoes, including the lids, leaving a wall about 1/4-inch thick, and transferring the scooped-out flesh to a large mixing bowl.
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15
Return the potato shells and lids to the bed of rock salt.
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16
Put the tray back in the hot oven, leaving the oven door open, to keep the potato shells and lids warm.
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17
With a potato masher, mash the potatoes.
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18
Season the mixture, to taste, with salt and generously with pepper.
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19
Add the creme fraiche or sour cream and the butter and mash to combine them with the potatoes.
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20
Stir in the ham, mushrooms, and 2 ounces of the cheese.
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21
Remove the baking sheet from the oven and turn the temperature up to 500 degrees F.
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22
Carefully spoon the mashed potato mixture back into each potato shell.
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23
Sprinkle the remaining cheese on top of the filling on each potato.
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24
Leave the lids unfilled, placing them around the side of the tray.
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25
Return the tray to the oven and continue baking the potatoes until they are heated through and the cheese has melted, 7 to 10 minutes.
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26
If the cheese has not browned, turn on the broiler and broil the potatoes briefly, keeping a close eye on them to prevent burning.
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27
Using tongs, transfer the potatoes to a heated serving platter or individual plates.
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28
Place the lids on top of the potatoes at an angle, inserting 1 end partway into the filling to secure them.
