Preheat oven to 375°F (190°C). Peel and thinly slice potatoes to a 1/8-inch thickness using a mandoline for best results. Submerge slices in cold water to prevent browning.

In a saucepan over medium heat, melt butter. Add minced onion and garlic, sautéing until fragrant. Whisk in flour to form a roux and cook for one minute. Gradually pour in heavy cream and milk, whisking continuously until smooth. Bring to a simmer and cook until thickened, about 5 minutes.

Remove the sauce from heat. Stir in the shredded Gruyère, sharp cheddar, thyme, nutmeg, salt, and pepper. Continue stirring until the cheese is fully melted and the sauce is creamy.

Grease a 9x13 inch baking dish. Drain the potatoes and pat them completely dry. Arrange a third of the potatoes in an overlapping layer in the dish. Cover with a third of the cheese sauce. Repeat for two more layers, finishing with a layer of sauce on top.

Cover the dish with foil and bake for 45 minutes. Remove the foil and bake for an additional 25-30 minutes, until the top is golden brown and bubbly, and the potatoes are tender.

Let the scalloped potatoes rest for 10-15 minutes before serving to allow the sauce to set.