Begin by peeling your russet or Yukon gold potatoes, if desired. Carefully slice them into thin rounds, about 1/8 to 1/4 inch thick, ensuring even cooking throughout. Arrange the sliced potatoes in a bowl of cold water to prevent browning while you prepare the creamy sauce.

In a medium saucepan, melt 3 tablespoons of butter over medium heat. Once melted, add the chopped onions and sauté for about 3 minutes until they become translucent and fragrant.

Stir in the minced garlic and fresh thyme, allowing them to cook for an additional minute.

Sprinkle the all-purpose flour into the onion mixture, stirring continuously for about 1 minute. Gradually pour in 2 cups of heavy cream, whisking to combine and prevent lumps. Continue stirring until the sauce thickens slightly and is smooth and creamy, roughly 3-4 minutes.

Add salt, pepper, and cayenne pepper to the cream sauce, adjusting to taste. Then, mix in 2 cups of shredded cheddar cheese until melted and fully incorporated.

Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with butter or oil to prevent sticking. Start layering the sliced potatoes in the dish, followed by a generous pour of the creamy cheese sauce. Repeat this process until all potatoes and sauce are used.

Cover the baking dish tightly with aluminum foil and place it in the oven. Bake for 40 minutes. Carefully remove the foil and continue baking for an additional 20 minutes until the cheese is bubbly and golden brown.

Once done, remove the dish from the oven and let it cool slightly. Garnish with freshly chopped chives, thyme, or parsley before serving.