Beer and Gruyère Croquettes

These are a lovely hors d’oeuvres, crunchy outside, creamy and savory inside. And beer, which is so important in Alsatian cooking, is a major component here. The bitterness of the beer, which is reduced to a syrup, is perfectly balanced by the caramelized onion. The key here is to allow as much moisture from the potato as possible to escape after cooking, so cut the potatoes in half lengthwise as soon as they’re out of the oven and allow them to cool. The fine “double-zero” flour is prefera