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["Preheat oven to 400 degrees.", "Place potatoes on a rimmed baking sheet and bake for 1 1/2 hours or until tender when pierced with a knife.", "Let stand until cool enough to handle.", "Reduce the oven temperature to 350 degrees.", "Meanwhile, in a medium skillet, melt 1 tablespoons of the butter in the olive oil.", "Add the sliced cremini mushrooms and cook over moderately high heat, stirring occasionally, until thery are golden brown around the edges and tender, about 4 minutes.", "Season generously with salt and black pepper and transfer the mushrooms to a large bowl.", "Cut off the top fourth of each potato.", "Scoop the flesh from the tops of the potatoes into the bowl with the sauteed mushroom, discard the skins.", "Scoop the floesh from the potatoes into the bowl, leaving a 1/4"" of shell.", "Place the potato shells on the baking sheet.", "While the potato flesh is still warm, add the remaining 7 tabsp of butter, 1 cup of the cheddar cheese, 1/2 cup of the Parmigiano-Reggiano cheese, the sour cream and 3 tbsp of the snipped chives to the bowl.", "Using a potato masher, whisk or pastry blender, mash the filling until the potatoes are almost smooth, the cheese is incorporated and the butter has melted.", "Season the potato filling with salt and pepper.", "Mound the mushroom-and-cheese filling into the empty potato skins and top with the remaining 1 cup of shredded cheddar cheese and 2 tablespoons of grated Parmigiano-Reggiano cheese.", "Bake the stuffed potatoes for 30 minutes, until the filling is heated through and the cheese on top is lightly golden.", "Sprinklethe potatoes with the remaining 1 tablespoons of snipped chives and serve immediately.", "To make ahead: The stuffed potatoes can be covered and refrigerated overnight.", "Bring to room temperature before baking."]
