1. Preheat oven to 300 F. Butter a 9-inch x 13-inch baking dish.
2. In a small skillet over medium heat, cook the bacon until barely crisp, about 4 minutes. Drain on paper towels, then transfer to large bowl. Stir in heavy cream, crème fraiche, garlic and thyme. Add salt and pepper to taste.
3. Slice potatoes and kohlrabi into very thin slices using a mandoline, food processor or sharp knife. Add slices to cream mixture. Toss gently to mix.
4. Spread mixture in an even layer in the baking dish. Sprinkle with grated cheese, if desired. Bake for 15 minutes.
5. Increase oven to 325 F and bake for 1 hour and 15 minutes or until potatoes are tender and top is nicely browned.
6. Remove from oven and let gratin sit for 10 minutes before serving.