Preheat the oven to 450 degrees. Put the potatoes on a cutting board. 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 into the potato about 1/8 inch all the way around. Spread a bed of rock salt about 1/4 inch thick on a baking tray large enough to hold the potatoes without touching. Place the potatoes lid sides up on the rock salt. Bake the potatoes until they are tender enough to be pierced easily with a wooden skewer, about 45 minutes. Remove the baking sheet from the oven, leaving the oven on, and let the potatoes cool slightly on the bed of rock salt. Meanwhile, heat a saut pan over high heat and add the olive oil. When the oil is hot enough to flow freely, add 2 tbsp. of the butter. When the butter starts to foam, add the ham and mushrooms and saut, stirring frequently, for 2 to 3 minutes. Transfer the ham and mushrooms to a small mixing bowl and set aside. Holding each hot potato with a kitchen towel cut off the lid with a sharp knife, following the scoring mark. Using a sharp-edged tablespoon scoop the flesh from the potatoes and the lids, leaving a wall about 1/4 inch thick, transferring the potato flesh to a large mixing bowl; set aside. Return the potato shells and lids to the baking tray with rock salt and return to the hot oven, leaving the oven door open, to keep the potato shells and lids warm. Mash the potatoes with a potato masher and season with salt and generously with pepper. Add the crme Fraiche or sour cream and the remaining butter and continue mashing until thoroughly blended. Stir in the ham, mushrooms, and 2 oz. of the cheese. Remove the baking sheet from the oven and turn the oven temperature up to 500 degrees. Carefully spoon the potato mixture back into each potato shell; the mixture will mound above the rim of each potato. Sprinkle the remaining cheese on top of the filling. Leave the lids unfilled, placing them around the side of the tray. Return the baking tray to the oven and bake until the potatoes are heated through and the cheese has melted, 7-10 minutes. To further brown the cheese, briefly use the broiler, keeping close watch to prevent burning. Carefully transfer the potatoes to a heated serving platter or individual plates. Place the lids on top of the potatoes at an attractive angle, partially inserting an end into the filling to secure each lid. Serve immediately.
Ready in 1 hour and 20 min