Tartiflette is a rich, super-cheesy (and super-delicious!) potato gratin from the French Alps.
Preheat the oven to 400°. In a medium saucepan, cover the potato with cold water and bring to a boil. Reduce heat to medium. Cook until just tender, about 25 minutes. Let cool slightly. Peel and cut into 1/4-inch-thick slices.
Meanwhile, in a large skillet, melt the butter over medium heat. Add the onion and 2 tsp. kosher salt. Cook, stirring occasionally, until the onion begins to brown, 8 to 10 minutes. Stir in the sugar, then half of the wine, scraping up the browned bits from the bottom of the pan. Cook, stirring often, until the liquid is reduced and the onion is golden, 8 to 10 minutes. Add the remaining wine. Cook, stirring occasionally and scraping the bottom of the pan, until the onion is deep brown and jammy, 8 to 10 minutes. Stir in the sage. Remove from heat.
Arrange half of the potato slices in a medium baking dish; season with kosher salt and pepper. Top with half of the onion. Repeat with the remaining potato slices and onion. Drizzle with the crème fraîche. Top with the cheese. Bake until golden-brown and bubbling at the edges, about 15 minutes. Serve immediately.
Serve this side dish with Angie Mar's Bone-In Rib Eye with Blackberries, Garlic & Prawn Butter