Step 1: Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking dish generously with butter.
Step 2: In a large saucepan, melt the butter over medium heat. Add the minced onion and cook for 3-4 minutes until translucent. Stir in the garlic and cook for another 60 seconds until fragrant.
Step 3: Whisk in the flour to create a roux and cook for 2 minutes to remove the raw flour taste, ensuring the mixture does not brown.
Step 4: Slowly whisk in the milk and heavy cream, a little at a time, to ensure a smooth sauce. Continue cooking and stirring until the sauce thickens and reaches a light boil.
Step 5: Remove the sauce from heat and stir in the salt, pepper, thyme, and nutmeg.
Step 6: Arrange half of the potato slices in an even layer in the prepared baking dish. Pour half of the cream sauce over the potatoes.
Step 7: Add the remaining potato slices in a second layer and top with the remaining cream sauce. Press down slightly to ensure all potatoes are submerged.
Step 8: Cover the dish tightly with aluminum foil and bake for 45 minutes. Remove the foil and bake for an additional 30 minutes, or until the potatoes are tender and the top is golden brown.
Step 9: Let the dish rest for 10-15 minutes before serving to allow the sauce to set.