Preheat your oven to 375°F. While the oven heats, slice the russet potato into 1/4-inch rounds—a mandoline or sharp knife works well here. Arrange the potato slices in a single-serving baking dish (a 2-cup ramekin or small gratin dish works perfectly) and set aside. I find that uniform potato thickness is key to even cooking, so take a moment to get those slices consistent.

Melt the butter in a small saucepan over medium heat. Once foaming, sprinkle the flour over the butter and stir constantly for about 2 minutes to cook out the raw flour taste—this is your roux, and it's what will thicken the sauce. Gradually whisk in the milk, stirring until the mixture is smooth and free of lumps, then continue cooking for another 2 minutes until it thickens enough to coat the back of a spoon.

Remove the saucepan from heat and stir in the 1/2 cup sharp cheddar cheese, salt, pepper, and garlic powder until the cheese is completely melted and the sauce is smooth. I like sharp cheddar because it gives the sauce more depth and complexity than mild varieties—it really elevates a simple potato dish. Taste and adjust seasoning if needed.

Pour the cheese sauce from Step 3 evenly over the potato slices in your baking dish, making sure the sauce coats all the potatoes. Bake uncovered at 375°F for 25 minutes until the potatoes are just becoming tender and the sauce is bubbling around the edges.

Remove the dish from the oven and sprinkle the remaining 1/4 cup cheddar cheese evenly over the top. Return to the oven and bake for 20 minutes more until the potatoes are fork-tender and the cheese topping is golden and melted. The total cooking time of 45 minutes ensures the potatoes cook through completely while the sauce stays creamy.