Preparing the Potatoes

Peel and slice potatoes into ⅛-inch thick rounds

Optional: blanch slices in boiling water for 2–3 minutes, then drain and pat dry

Making the Cheese Sauce

Melt butter in a medium saucepan over medium heat

Stir in flour and cook 1–2 minutes until lightly golden

Gradually whisk in warm milk and heavy cream; simmer until slightly thickened

Season with salt, pepper, and nutmeg

Remove from heat and stir in 1 cup Gruyère and ¼ cup Parmesan until melted and smooth

Assembling the Casserole

Preheat oven to 375°F (190°C)

Grease a 9×13-inch baking dish

Layer half of the potato slices in the dish

Pour half of the cheese sauce over potatoes

Repeat with remaining potatoes and sauce

Adding Topping

Combine bread crumbs, melted butter, and Parmesan

Sprinkle evenly over the top; optionally add extra Gruyère

Baking

Cover casserole with foil and bake 25 minutes

Remove foil and bake 20–25 minutes more until golden and bubbly

Let rest 10 minutes before serving