Make Cheese Sauce:

Cup of Yum
1. In a medium saucepan, melt the butter over medium low heat. As it's melting, add in the thyme leaves and garlic and let cook until the garlic is fragrant, about 2 minutes.
2. Sprinkle in 4 tbsp of flour, a little at a time, while continuously whisking. Don’t dump it all at once—this helps prevent clumps. Cook for about 1 minute to cook out the raw flour.
3. Gradually pour in the cream, whisking constantly. The mixture will seize up at first and turn into a paste, but keep whisking and adding cream until it forms a smooth, liquidy sauce. Allow it to simmer gently over medium heat and reduce until it’s thick enough to coat the back of a spoon (nappé). Because we're not using a lot of liquid compared to the roux, it won't take very long, around 3 minutes.
4. Turn off the heat and stir in ½ cup of shredded gruyere and ½ cup of shredded white cheddar, a handful at a time. Stir until melted before adding another handful. Be careful not to add the cheese while the heat is on to avoid a broken, gritty, greasy cheese sauce. Taste and season with salt and pepper.

Layer and Bake

1. Preheat oven to 350 degrees F. Layer the sliced potatoes in a baking dish, covering the entire bottom of the dish with one single layer.
2. Pour a layer ⅓ cup of the cream sauce over the first layer of potatoes, then cover evenly with a mixture of the cheddar and gruyere. Repeat until you've reached almost the top of your baking dish, about 3-4 layers ending with the last bit of cheese sauce. Cover the top with the last 1 cup of your cheese.
3. Cover with aluminum foil and Bake in the oven for 30 minutes. Remove the foil and continue baking for an additional 30 minutes, or until the top is golden brown and bubbly.
4. Let cool for 5-10 minutes before serving.