In Northern Italy, this is called nervetti. Mr. Curtola had it in Venice, with onions and a bottle of vinegar plonked alongside. His presentation at I Cavallini is more delicate. The onions are briefly immersed in the liquid from chive blossoms he pickled last spring, lending a pang of grassy sweetness. And those tiny blobs? That’s the gelatin that rises to the surface of the terrine, strained and clarified, ready to dissolve into a burst of bouillon.