Prepare the Oven and Baking Dish:

• Preheat your oven to 375°F (190°C).​

• Grease a 9×13-inch (23×33 cm) baking dish with butter or non-stick cooking spray.​

Slice the Potatoes and Onion:

• Using a mandoline slicer or a sharp knife, thinly slice the potatoes to about 1/8-inch thickness.​

• Slice the onion into thin rings.​Allrecipes

Make the Creamy Sauce:

• In a medium saucepan over medium heat, melt the butter.​

• Add the minced garlic and sauté for about 1 minute until fragrant.​

• Whisk in the flour and cook for 1-2 minutes to eliminate the raw flour taste.​

• Gradually whisk in the milk and heavy cream, ensuring no lumps form.​

• Add the salt, pepper, and nutmeg. Continue to cook, stirring frequently, until the mixture thickens, about 5-7 minutes.​Reddit

Assemble the Gratin:

• Arrange half of the sliced potatoes in an even layer in the prepared baking dish.​Allrecipes

• Top with half of the sliced onions.​Allrecipes+1Allrecipes+1

• Pour half of the creamy sauce over the potatoes and onions, spreading it evenly.​Allrecipes

• Sprinkle half of the shredded cheese over the sauce.​

• Repeat the layers with the remaining potatoes, onions, sauce, and cheese.​

Bake:

• Cover the baking dish with aluminum foil and bake in the preheated oven for 45 minutes.​

• After 45 minutes, remove the foil and continue baking for an additional 20-30 minutes, or until the top is golden brown and the potatoes are tender when pierced with a fork.​

Serve:

• Remove from the oven and let the gratin rest for about 10 minutes before serving.​Reddit+2Simply Recipes+2Spoon Fork Bacon+2

• Garnish with fresh thyme leaves if desired.