Preheat Oven:
Preheat your oven to 375°F (190°C). Butter a baking dish (around 9×9 inches or similar).
Prepare Cheese Sauce:
In a saucepan over medium heat, melt the butter.
Add chopped onions and garlic, sauté until softened (about 3-4 minutes).
Stir in the flour and cook for 1 minute to make a roux.
Gradually whisk in milk (and cream, if using), stirring constantly to avoid lumps.
Simmer for a few minutes until thickened.
Remove from heat, season with salt and pepper, and stir in 1½ cups of the grated cheddar until melted and smooth.
Assemble the Gratin:
Layer half the sliced potatoes at the bottom of the baking dish.
Sprinkle half the ham over the potatoes.
Pour half the cheese sauce on top.
Repeat with the remaining potatoes, ham, and cheese sauce.
Sprinkle the remaining ½ cup cheddar on top.
Bake:
Cover with foil and bake for 30 minutes.
Remove foil and bake uncovered for an additional 25–30 minutes, or until golden and bubbly, and potatoes are fork-tender.
Garnish & Serve:
Let rest for 5–10 minutes before serving.
Garnish with fresh thyme or parsley if desired.
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