Scalloped Potatoes

Method

1. slice10 min
   1/8 inch, even
   potatoes to one consistent thickness — the bake is timed by the slice, and one thick layer among thin ones is the crunchy bite that gets the whole dish called underdone; do not rinse, the surface starch is a thickener you already paid for
2. whisk8 min
   medium, roux then milk
   flour into melted butter for a minute until it smells baked rather than raw, then warm milk in a slow stream — cold milk seizes a roux into lumps; warm milk lets it open into a sauce that coats a spoon
3. prep8 min
   layers into a buttered dish: potatoes, onion + garlic, salt + pepper, sauce, repeated to the top with sauce as the final coat — every layer seasoned, because a bland middle cannot be fixed from the surface
4. bake40 min
   375°F, covered
   foil on — the trapped steam is what actually cooks the potatoes through; uncovered from the start, the top browns long before the center gives
5. bake20 min
   375°F, uncovered, until deeply golden
   foil off (cheese goes on now if using) until the top blisters bronze and a knife slides through the middle with no resistance
6. rest10 min
   before serving — the sauce is liquid at oven temperature and thickens as it cools; cut early, the layers slide and the dish pools