The Method:
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Get Ready: Preheat your oven to 375°F (190°C) and generously grease a 9×13-inch ceramic or glass baking dish with butter.
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Build the Cream Base: In a medium saucepan, melt the 3 tablespoons of butter over medium heat. Add the minced garlic and cook for about 1 minute until fragrant but not browned. Whisk in the flour to form a smooth paste (a roux) and cook for another minute to remove the raw flour taste.
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Infuse the Liquid: Slowly pour in the whole milk and heavy cream, whisking constantly to eliminate any lumps. Bring the mixture to a gentle simmer. Add the salt, black pepper, thyme, and a pinch of nutmeg. Simmer for 2 to 3 minutes until the sauce begins to lightly thicken, then remove from the heat. Stir in 1 cup of the sharp cheddar until completely melted.
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Layer the Dish: Arrange half of your sliced potatoes in the prepared baking dish, overlapping them slightly to create a solid layer. Pour half of the warm cheese sauce evenly over the top. Sprinkle with the remaining cup of cheddar cheese. Layer the remaining potatoes on top, and pour the rest of the cream sauce over them, pressing down gently with a spatula so the liquid submerges the spuds.
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The Two-Stage Bake: Cover the baking dish tightly with aluminum foil (this traps the steam to cook the potatoes through). Bake covered for 45 minutes. 6. The Golden Crust: Remove the foil, scatter the Gruyère or Parmesan cheese across the top, and return to the oven uncovered for an additional 25 to 30 minutes. The dish is ready when the potatoes are completely tender when pierced with a fork, the cream is bubbling at the edges, and the top is a spectacular, blistered golden-brown.
The Golden Rule of Serving
As tempting as it is to scoop into the pan the absolute second it clears the oven, let the dish rest on the counter for 15 minutes before serving. When hot out of the oven, the cream sauce will appear loose. Letting it rest allows the starch in the potatoes to cool slightly and reabsorb the residual liquid, tightening the layers into a perfect, spoonable consistency that holds its shape beautifully on the plate
