Step 1Select potatoes that are as similar in size and as evenly cylindrical as possible. Put them on a baking sheet in a cold oven and turn the oven to 400 degrees. Bake until tender but not soft, about 50 to 55 minutes. The potatoes should be easy to penetrate with a knife, but not mushy.
Step 2Remove the potatoes and set them aside until they are cool enough to handle. Meanwhile, butter an 8-inch-square earthenware, glass or enameled cast-iron baking dish.
Step 3Trim away the pointed ends of the potatoes and remove the peel, taking care to keep the flesh intact. Cut in roughly one-fourth-inch to one-third-inch slices and arrange the slices in a single, overlapping layer in the baking dish. Use the smallest slices in the back row (it will be mostly covered) and to fill in spaces. Season with salt and a generous amount of freshly ground black pepper.
Step 4Beat together the whipping cream and creme fraiche until smooth, about 45 seconds, and drizzle it evenly over the top of the potatoes. There should be enough cream to come halfway up the potatoes, but not so much that they're swimming in it. Scatter the Gruyere over the top and dot with butter.
Step 5Bake until the cheese is melted and the cream is bubbling and golden, 15 to 20 minutes. Serve immediately.