What's the difference between potatoes au gratin and scalloped potatoes?

Scalloped potatoes use a flour-thickened milk sauce (a roux). Potatoes au gratin is the high society version—it dispenses with flour entirely and says 'Let them eat cream!' With the addition of cheese, you're not just thickening; you're transforming. It's alchemy with dairy.

Why did my gratin become curdled?

Curdling happens when using milk instead of heavy cream. The fat content in cream keeps everything luscious and cohesive. Always use heavy cream, possibly cut with a bit of chicken broth. Toss your cheeses with cornstarch to prevent clumping and help emulsify the sauce.

Which potatoes should I use for gratin?

Use starchy potatoes like russets, Maris Piper, King Edward, or Sebago. These varieties break down just enough to meld seamlessly with the sauce while still holding their form. Avoid waxy varieties as they refuse to play nicely with the cream.