How to Make It
Preheat the oven to 400°. Place the 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°. Meanwhile, in a medium skillet, melt 1 tablespoon of the butter in the olive oil. Add the sliced cremini mushrooms and cook over moderately high heat, stirring occasionally, until they 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 sautéed mushrooms; discard the skins. Scoop the flesh from the potatoes into the bowl, leaving a 1/4 inch of shell. Place the potato shells on the baking sheet.
While the potato flesh is still warm, add the remaining 7 tablespoons of butter, 1 cup of the cheddar cheese, 1/2 cup of the Parmigiano-Reggiano cheese, the sour cream and 3 tablespoons 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 black 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. Sprinkle the potatoes with the remaining 1 tablespoon of snipped chives and serve immediately.