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

Wash, peel, and thinly slice the russet potatoes to approximately 1/8-inch thickness. For best results, use a mandoline slicer for uniform cuts. Place the sliced potatoes in cold water to prevent browning while you prepare the sauce.

In a medium saucepan over medium heat, melt the butter. Add the diced onion and cook until softened and translucent, about 5-6 minutes. Add the minced garlic and cook for an additional 30 seconds until fragrant.

Sprinkle the flour over the onion mixture and whisk continuously for 1-2 minutes to create a roux. This will thicken your cream sauce.

Gradually whisk in the warmed milk and heavy cream. Continue whisking until the mixture is smooth with no lumps.

Add the dried thyme, nutmeg, salt, and black pepper. Simmer gently for 3-4 minutes until the sauce thickens enough to coat the back of a spoon.

Drain the potato slices and pat them dry with paper towels. Layer about one-third of the potatoes in the bottom of the prepared baking dish, slightly overlapping the slices.

Pour approximately one-third of the cream sauce over the first layer of potatoes. Repeat the layering process two more times with the remaining potatoes and sauce.

Cover the baking dish tightly with aluminum foil and bake for 45 minutes.

Remove the foil and bake for an additional 15-20 minutes, until the top is golden brown and the potatoes are fork-tender.

Let the scalloped potatoes rest for 10-15 minutes before serving. This allows the sauce to set and makes it easier to serve. Garnish with fresh chopped parsley.