Preheat your oven to 350°F (175°C).

Thinly slice the Russet and Yukon Gold potatoes into 1/8-inch rounds.

Finely mince the onion and garlic, and chop fresh thyme if using.

Melt butter in a medium saucepan, add onion and garlic, and sauté for 3-5 minutes until softened.

Stir in the flour to create a roux, cooking for 1 minute.

Gradually whisk in the milk, simmer for 1-2 minutes, then stir in cheese, nutmeg, salt, and pepper.

Butter a large baking dish, add a small amount of cheese sauce, layer half of the sliced potatoes, then half of the cheese sauce.

Add the remaining potatoes and top with the rest of the cheese sauce.

Bake in the preheated oven for 45-60 minutes until potatoes are tender and the top is golden.

For a crunchier crust, broil for 2-3 minutes at the end of baking.

Let it rest for 10 minutes before serving and garnish with fresh herbs.