The secret to perfect potatoes au gratin lies in meticulous layering of thinly and uniformly sliced (2-3mm), appropriate medium-starch potato varieties like Maris Piper or Desiree, which provide structure and starch. This is combined with a properly seasoned, rich cream or béchamel sauce, aggressively flavoured before baking, allowing the potatoes to absorb its depth.
Finally, slow baking at a moderate temperature ensures the potatoes fully cook, absorb the liquid, and meld beautifully, resulting in a cohesive, deeply flavoured gratin with a desirable golden crust.