1. Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with butter or non-stick spray.
2. In a medium saucepan, melt the butter over medium heat. Add the sliced onion and cook until softened and translucent, about 5 minutes. Stir in the minced garlic and cook for another 30 seconds until fragrant.
3. Sprinkle the flour over the butter and onions, whisking constantly to form a roux. Cook for 1-2 minutes, but don’t let it brown—that’s key to keeping the sauce smooth and light.
4. Gradually whisk in the milk and cream, blending well to avoid lumps. Continue to cook, stirring frequently, until the sauce thickens enough to coat the back of a spoon, about 5-7 minutes. Season with salt, pepper, and thyme.
5. Remove the sauce from heat and stir in 1½ cups of the shredded cheese until melted and smooth.
6. Arrange half of the potato slices in an even layer in the prepared baking dish. Pour half of the cheese sauce over the potatoes, spreading it gently with a spatula. Repeat with the remaining potatoes and sauce.
7. Sprinkle the remaining ½ cup of cheese evenly on top.
8. Cover the dish tightly with foil and bake for 45 minutes. Then remove the foil and bake for an additional 20-25 minutes, or until the top is golden and bubbly, and the potatoes are tender when pierced with a fork.
9. Let the scalloped potatoes rest for 10 minutes before serving. This helps the sauce thicken slightly and makes slicing easier.

I still remember the first time I nailed this step-by-step after oversalting a pot of chili back when I was twelve. That little kitchen disaster taught me patience and the magic of layering flavors and textures—skills that made all the difference with these scalloped potatoes. Watching Eli dig in without negotiation makes all the effort worthwhile now.