Put the oven rack in center of the oven and preheat to 350 degrees F.
In a saucepan over low heat, add the cream and 3/4 cup of the walnuts. Bring just to a simmer and then keep warm over lowest heat while building the potato layers.
In a large, lightly greased, 9 by 13-inch baking dish, arrange 1/3 of the potato slices covering the bottom of the dish, overlapping slightly. Generously season with salt and pepper and sprinkle with a little sage, 1/3 of the Gruyere and a ladle of warmed walnut cream mixture. Repeat 1 more time the exact same way. Layer for a 3rd and last time with just the potatoes, making sure to arrange them in nice straight even rows. Then cover the top of the potatoes with the remaining warmed walnut cream and top with the last 1/3 cup of the cheese. Sprinkle with salt and pepper, to taste.
Cover with foil and bake for 30 minutes. Reduce the oven temperature to 325 degrees and remove the foil. Continue to bake until the top is just starting to become golden, about 20 minutes more.
Remove from the oven and do your best to let the gratin rest for 15 to 20 minutes before serving. Eat and enjoy!
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