Preheat your oven to 375°F (190°C). This ensures your scalloped potatoes will bake evenly and come out perfectly golden.

Peel and slice your russet potatoes into ¼-inch thick rounds. It's crucial to slice them evenly; this allows for consistent cooking. Don’t worry if they aren’t perfect; rustic is the name of the game.

In a baking dish, begin layering half of the potato slices and then add half of the sliced onion rings on top. Add a sprinkle of salt, a dash of pepper, and some fresh thyme leaves for flavor.

In a saucepan, melt the butter over medium heat. Whisk in the flour to create a roux, cooking it for about a minute until it’s lightly golden. Gradually add the whole milk, ensuring there are no lumps. Continue to whisk until the mixture thickens. Stir in the garlic powder and half of the shredded cheese until melted.

Pour half of the creamy mixture over the first layer of potatoes and onions. Repeat the layering process using the remaining potatoes and onions. Finish it off by pouring the rest of the cream mixture on top.

Sprinkle the remaining cheese across the top. This will get beautifully browned and add a delightful crust.

Cover the baking dish with foil and bake for 45 minutes. Then, remove the foil and bake for an additional 15-20 minutes or until the top is golden brown and the edges are bubbling.

Let your scalloped potatoes cool for about 10 minutes before serving. This will allow everything to set and make for easier serving. Enjoy the masterpiece you’ve created!