Calçots amb Romesco (Grilled Onions with Romesco Sauce)

Calçots, which originated in the town of Valls in Catalonia, Spain, are a particular kind of onion, hilled up as they grow to have a longer section of thick, tender white stalk. Calçots resemble leeks, but don’t taste like them. At their best for only three weeks each spring, they’re grilled on a rack over hot coals until they’re completely tender insid…