Special equipment: a mandoline, optional
Preheat the oven to 375 degrees F. Stir together the soy sauce and mushroom powder in a small bowl and set aside.
Add the cream, garlic and thyme sprigs to a medium pot and set over medium-high heat. Bring to a simmer, cover and remove from the heat. Let stand 15 to 30 minutes.
Remove the garlic and thyme from the cream, then whisk in the creme fraiche, nutmeg and soy-mushroom mixture until well incorporated; season with salt and pepper.
Assemble the gratin by putting half of the potatoes in a single overlapping layer in the bottom of a 2-quart baking dish. Coat with half of the cream mixture and sprinkle on 1 cup of the Gruyere. Repeat with the remaining potato slices, cream mixture and 1 cup Gruyere. Cover with foil and transfer the dish to a baking sheet.
Bake until the potatoes are easily pierced with a knife, about 1 hour. Remove the foil and continue to bake until the top is golden brown and bubbling, 30 minutes. Let sit 30 minutes before serving.
If you can't find porcini powder, you can pulverize dried porcini mushrooms in your food processor or spice grinder; or try other dried mushrooms here.