1. Preheat oven to 375°F and generously butter a 9×13-inch baking dish.

2. Slice potatoes uniformly 1/8-inch thick using mandoline or sharp knife, then soak in cold water for 10 minutes.

3. Warm milk in microwave for 60-90 seconds until lukewarm.

4. Melt butter in large saucepan over medium heat, add flour and whisk constantly for 2-3 minutes until light golden.

5. Gradually add warm milk in steady stream while whisking continuously to prevent lumps.

6. Add heavy cream and continue whisking until mixture thickens enough to coat back of spoon, about 5-7 minutes.

7. Remove from heat and stir in 2 cups cheddar cheese, all Gruyère, garlic powder, onion powder, mustard powder, nutmeg, salt, and pepper until smooth.

8. Pat potato slices completely dry with kitchen towels.

9. Arrange one layer of overlapping potato slices in buttered baking dish.

10. Pour 1/3 of cheese sauce over potatoes, spread evenly, and sprinkle with fresh thyme.

11. Repeat layering twice more, ending with cheese sauce completely covering top layer.

12. Cover dish tightly with aluminum foil and bake for 45 minutes.

13. Remove foil, sprinkle remaining 1 cup cheddar cheese over top, and bake uncovered 20-30 minutes until golden brown and bubbling.

14. Test doneness by inserting knife through center—it should slide through easily with no resistance.

15. Let rest at room temperature for 15-20 minutes before serving to allow sauce to thicken and layers to set.

16. Garnish with fresh parsley and serve warm.