Lightly salt the potato slices and distribute them in a gratin dish. Heat the butter and sauté the garlic and sage in pieces. Pour in the broth and cream, season with plenty of pepper and a little chili. Bring to a boil.
Mix the potato slices with the sauce. Grate the parmesan on top and sprinkle flakes of sour cream on top.
Bake in the oven on the second rack from the top at 200 ° C for 50 minutes. Then another 15 minutes at 180 ° C.