Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish (or an equivalent 3-quart dish) with butter or cooking spray.

Ensure the potatoes are uniformly sliced. If slicing by hand, a mandoline is recommended for the best texture. If sliced ahead of time, keep them submerged in cold water to prevent browning, then drain thoroughly and pat dry before layering.

In a medium saucepan, melt the 1 tablespoon of butter over medium heat. Add the finely minced onion and sauté for about 3-5 minutes, until translucent and softened. Stir in the minced garlic and cook for 1 minute until fragrant. Do not let the garlic brown.

Whisk in the 1/4 cup of flour and cook for 1-2 minutes, stirring constantly, to create a roux. This step cooks out the raw flour taste.

Gradually pour in the whole milk and heavy cream while whisking continuously to prevent lumps. Bring the mixture to a gentle simmer, stirring frequently. The sauce should begin to thicken significantly within a few minutes.

Remove the saucepan from the heat. Stir in the 1 teaspoon of salt, 1/2 teaspoon of black pepper, and the 1/4 teaspoon of nutmeg. Taste the sauce and adjust seasonings as needed.

Spread a thin layer of the cream sauce (about 1/2 cup) on the bottom of the prepared baking dish. Arrange one-third of the sliced potatoes in a slightly overlapping pattern over the sauce layer. Sprinkle a generous pinch of salt and pepper over the potatoes.

Pour about one-third of the remaining cream sauce evenly over the potato layer. Sprinkle 1/2 cup of the shredded cheese over the sauce. Repeat the layering process two more times: Second potato layer, followed by seasonings, another third of the sauce, and 1/2 cup of cheese. Third and final potato layer, followed by the remaining sauce. (Reserve the final 1 cup of cheese for the very top.)

Cover the baking dish tightly with aluminum foil. Bake for 50 minutes. This steaming process allows the potatoes to cook through without burning the top.

Carefully remove the baking dish and take off the foil. Test the potatoes with a fork; they should be nearly tender. Sprinkle the remaining 1 cup of cheese evenly over the top layer. Return the dish to the oven, uncovered, and bake for an additional 20 to 30 minutes, or until the cheese is bubbly, golden brown, and the potatoes are completely fork-tender.

Remove the scalloped potatoes from the oven. Allow them to rest for 10 to 15 minutes before serving. This rest time allows the sauce to settle and firm up, making serving easier and ensuring a creamier result.