Preheat your oven to 400°F (200°C). Generously butter a 9x13 inch baking dish.

Peel the potatoes and slice them into 1/8-inch thick rounds using a mandoline slicer for best results, or a very sharp knife. Place the sliced potatoes in a large bowl of cold water to prevent them from browning while you prepare the sauce.

In a large saucepan or Dutch oven, melt the butter over medium heat. Add the finely chopped onion and cook until softened and translucent, about 4-5 minutes. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the onions and garlic, and whisk constantly for 1-2 minutes to cook out the raw flour taste. This creates a roux.

Gradually pour in the warm milk while whisking continuously to prevent lumps. Once all the milk is incorporated, whisk in the heavy cream. Bring the mixture to a simmer, stirring frequently, until the sauce thickens enough to coat the back of a spoon, about 5-7 minutes.

Remove the saucepan from the heat. Stir in the salt, pepper, and nutmeg. Add 1 cup of the shredded Gruyère and all of the Parmesan cheese, stirring until the cheese is completely melted and the sauce is smooth.

Drain the potato slices thoroughly and pat them dry with paper towels. Add the dry potato slices to the saucepan with the cheese sauce and gently stir to coat every slice evenly.

Transfer the coated potato mixture into your prepared baking dish, spreading it into an even layer.

Cover the baking dish tightly with aluminum foil and bake for 45-50 minutes. The potatoes should be tender when pierced with a fork.

Remove the foil, sprinkle the remaining 1/2 cup of Gruyère cheese over the top. Return the dish to the oven, uncovered, and bake for another 20-25 minutes, or until the top is bubbly and golden brown.

Let the scalloped potatoes rest for at least 10-15 minutes before serving. This allows the sauce to set. Garnish with fresh chives or parsley if desired.