Kathleen Purvis offers the secret to making scalloped potatoes without curdling the sauce.

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Q. Why does the sauce for scalloped potatoes sometimes curdle?

A. Scalloped or au gratin potatoes are so simple. But it can be frustrating when the creamy sauce and cheese separate or look curdled. The good news is that the dish tastes just fine, even when it looks a little strange.

The curdling is caused by high heat, which is hard to avoid in an oven. So to keep the sauce together, tackle it before the dish goes in the oven. Instead of simply layering grated cheese and milk or cream with the potatoes, you need to make a smooth cheese sauce.

First, make a béchamel: Melt 3 tablespoons butter in a heavy saucepan over low heat. Whisk in 3 tablespoons all-purpose flour and cook, stirring constantly, for a couple of minutes. Whisk in 2 cups milk. Increase the heat and cook, whisking occasionally, until it’s smooth and a little thick, about 5 minutes.

Remove from heat and whisk in about 3 cups grated cheese, adding the cheese in small handfuls and stirring until it is completely melted before adding more. Season the sauce however you like, then continue with the dish.