Preheat your oven to 375°F (190°C). Generously butter a 9x13-inch baking dish and set aside.

Peel the potatoes and slice them into 1/8-inch thick rounds using a mandoline slicer or sharp knife. Keep slices in cool water to prevent browning while you work. Pro tip: Consistent thickness is key - thinner slices cook more evenly and create better layers.

In a large saucepan, melt 4 tablespoons of butter over medium heat. Whisk in the flour and cook for 2 minutes, stirring constantly to create a light roux. Pro tip: Don't let the roux brown - you want it pale and smooth for the best color in your finished dish.

Gradually whisk in the heavy cream and milk, adding about 1/2 cup at a time to prevent lumps. Continue whisking until the sauce is smooth and begins to thicken, about 5-7 minutes. Pro tip: Room temperature dairy prevents the sauce from breaking and creates a silkier texture.

Season with salt, pepper, nutmeg, and fresh thyme leaves. Remove from heat and stir in 1 cup of the cheddar cheese until melted and smooth.

Drain the potato slices and pat them dry with paper towels. Arrange half of the potato slices in overlapping layers in the prepared baking dish. Pro tip: Slightly overlapping slices create beautiful presentation and help the dish hold together when served.

Pour half of the cream sauce over the first layer of potatoes, making sure to get sauce between the slices. Add the remaining potatoes in another overlapping layer. Pro tip: Use a spoon to gently lift edges and let sauce flow underneath for even coverage.

Pour the remaining cream sauce over the top layer of potatoes. Sprinkle with the remaining cheddar cheese and all of the Gruyère cheese. Pro tip: Don't skip the Gruyère - it adds incredible nutty flavor and helps create that perfect golden top.

Cover tightly with foil and bake for 45 minutes. Remove foil and continue baking for 25-30 minutes until the top is golden brown and the potatoes are fork-tender. Pro tip: Test doneness by inserting a knife through all layers - it should slide in easily with no resistance.

Let rest for 10-15 minutes before serving to allow the sauce to set up properly. Pro tip: This resting time is crucial - it prevents the dish from being too liquidy when you serve it.