How to Make It
Preheat the oven to 300°. Butter a 9-by-13-inch glass baking dish. In a small skillet, cook the bacon over moderate heat, stirring, until barely crisp, about 4 minutes. Drain on paper towels, then transfer to a large bowl. Stir in the heavy cream, crème fraîche, garlic and thyme. Season the cream generously with salt and pepper.
Using a mandoline, thinly slice the potatoes and kohlrabi, then add to the cream and toss to mix. Spread the potatoes, kohlrabi and cream in an even layer in the prepared baking dish and bake for 15 minutes. Increase the oven temperature to 325° and bake for about 1 hour and 15 minutes longer, or until the potatoes are tender and the top is browned. Let the gratin stand for 10 minutes before serving.