Follow these steps to prepare your Kohlrabi and Potato Gratin Twist:

1. Preheat the oven to 375°F (190°C).
2. Prepare the vegetables: Peel and thinly slice the potatoes and kohlrabi, aiming for uniform slices for even cooking. Thinly slice the onion and mince the garlic.
3. Sauté the onions and garlic: In a large skillet, heat the olive oil over medium heat. Add the sliced onions and cook until they become translucent, about 5 minutes. Add the minced garlic and sauté for another 1-2 minutes until fragrant.
4. Make the creamy sauce: In a saucepan, combine the heavy cream and milk. Heat gently over medium heat until warm but not boiling. Stir in the nutmeg, salt, and black pepper.
5. Layer the gratin: Grease a baking dish with butter. Arrange a layer of potato slices on the bottom, followed by a layer of kohlrabi slices. Spread a portion of the sautéed onions and garlic over the layers. Sprinkle a portion of Gruyère cheese on top. Repeat the layers until all ingredients are used, finishing with a layer of cheese.
6. Pour the creamy sauce: Evenly pour the warm cream mixture over the layered vegetables, allowing it to seep through and coat each layer.
7. Top with Parmesan cheese: Sprinkle the grated Parmesan cheese over the top layer.
8. Bake the gratin: Place the baking dish in the preheated oven and bake for 45-60 minutes, or until the top is golden brown and the vegetables are tender when pierced with a fork.
9. Garnish and serve: Remove the gratin from the oven and let it cool slightly. Garnish with fresh thyme sprigs before serving.