Follow these detailed instructions to ensure your cheesy scalloped potatoes turn out perfectly every time. Don’t rush the process; each step contributes to the final, delicious result.

1. Preheat Your Oven and Prepare Baking Dish: Start by preheating your oven to 375°F (190°C). Grease a 9×13 inch baking dish with butter or cooking spray. This prevents sticking and ensures easy cleanup later. Having the oven preheated and the dish ready sets you up for a smooth cooking process.
2. Prepare the Potatoes and Onion: Wash and peel the russet potatoes. Thinly slice them to about 1/8 inch thickness. Uniformly thin slices are crucial for even cooking. You can use a mandoline for speed and consistency, or carefully use a sharp knife. Peel and thinly slice the yellow onion as well. Set both aside.
3. Sauté Aromatics: In a large, heavy-bottomed saucepan or Dutch oven, melt the butter over medium heat. Add the sliced onion and cook until softened and translucent, about 5-7 minutes. Stir occasionally to prevent browning. Add the minced garlic and cook for another minute until fragrant. Be careful not to burn the garlic, as it can become bitter.
4. Create the Roux: Sprinkle the all-purpose flour over the softened onions and garlic in the saucepan. Cook, stirring constantly with a whisk, for 1-2 minutes. This cooks out the raw flour taste and creates a smooth roux. The mixture should be slightly thickened and fragrant.
5. Gradually Add Milk and Cream: Slowly pour in the milk, whisking constantly to prevent lumps from forming. Continue whisking until the mixture is smooth. Gradually add the heavy cream, whisking to combine. The sauce will start to thicken as it heats.
6. Simmer and Thicken the Sauce: Bring the sauce to a gentle simmer over medium-low heat, stirring occasionally. Continue to simmer for 5-7 minutes, or until the sauce has thickened slightly and is smooth and creamy. Stirring prevents scorching on the bottom of the pan.
7. Incorporate Cheese and Seasoning: Remove the saucepan from the heat. Stir in 2 cups of the shredded cheddar cheese and all of the grated Parmesan cheese until melted and smooth. Season generously with salt and black pepper to taste. Remember that cheese is salty, so start with a moderate amount of salt and adjust as needed. If using, stir in the chopped fresh thyme.
8. Layer Potatoes and Sauce: Arrange about one-third of the sliced potatoes in a single layer at the bottom of the prepared baking dish. Pour about one-third of the cheese sauce evenly over the potatoes. Repeat layering twice more, ending with a layer of sauce on top. Ensure the potatoes are evenly distributed and covered in sauce for optimal cooking and flavor.
9. Top with Remaining Cheese: Sprinkle the remaining 1 cup of shredded cheddar cheese evenly over the top layer of sauce. This creates that beautiful golden, cheesy crust.
10. Bake to Golden Perfection: Bake in the preheated oven for 50-60 minutes, or until the potatoes are tender when pierced with a fork and the top is golden brown and bubbly. The baking time may vary slightly depending on your oven and the thickness of your potato slices. Check for tenderness after 50 minutes.
11. Rest Before Serving: Let the scalloped potatoes rest for 10-15 minutes before serving. This allows the sauce to set slightly and makes it easier to serve. Plus, it prevents burning your mouth!
12. Serve and Enjoy! Serve hot and enjoy the creamy, cheesy goodness of your homemade scalloped potatoes! They are the perfect side dish for almost any main course.