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Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with cooking spray or a bit of butter.
Prep the potatoes by washing, peeling (if desired), and slicing them into thin, even rounds, about 1/8 inch thick. A sharp knife or mandoline works well here. Set aside.
In a medium saucepan, melt the butter over medium heat. Add the chopped onion and cook for 3–4 minutes, stirring occasionally, until softened and just starting to turn golden.
Sprinkle the flour over the onions and stir well to coat. Cook this mixture for about 1 minute to get rid of the raw flour taste, stirring constantly so it doesn’t burn.
Slowly pour in the milk while whisking, breaking up any lumps. Continue to cook and whisk over medium heat until the sauce thickens and starts to bubble gently, 4–6 minutes.
Turn off the heat and stir in 1 cup shredded cheddar cheese, salt, pepper, garlic powder, and dried thyme or parsley if using. Taste and adjust seasoning if needed—the sauce should be nicely seasoned since it will flavor all the potatoes.
Spread about 1/3 of the sliced potatoes in an even layer on the bottom of the prepared baking dish. Sprinkle 1/3 of the diced ham over the potatoes.
Pour about 1/3 of the cheese sauce evenly over this layer, trying to cover as many potatoes as possible. It doesn’t have to be perfect; it will spread as it bakes.
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Repeat the layers two more times: potatoes, ham, then sauce, finishing with sauce on top. Gently shake the pan or tap it on the counter to help the sauce settle down into the layers.
If you’d like a cheesy top, sprinkle an extra 1/2 cup shredded cheddar cheese evenly over the casserole.
Cover the baking dish tightly with foil and bake in the preheated oven for 40 minutes. This helps the potatoes steam and get tender.
After 40 minutes, carefully remove the foil and continue baking for another 20–30 minutes, or until the potatoes are very tender when pierced with a fork and the top is bubbly and lightly browned.
Remove the casserole from the oven and let it rest for about 10–15 minutes before serving. This helps the sauce thicken slightly and makes it easier to scoop neat portions.
Scoop into bowls or onto plates and serve warm. Store any leftovers covered in the refrigerator for up to 3–4 days; they reheat well in the microwave or covered in the oven.
Variations & Tips
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