Fat content, mostly. Belly cuts like ventresca, oily fish like mackerel and sardines, and preparations packed in olive oil or butter all carry more fat, which means more flavor and a softer texture.

The belly cut of tuna or bonito. It has the highest fat content of any part of the fish, which gives it a soft, almost spreadable texture. It is the most expensive cut in most producers' ranges.

Mackerel, sardines, and salmon among the oily fish. Tuna belly, cod liver, and seafood pates among the specialty preparations. Smoked versions of any of these read richer still, because smoke concentrates flavor.

With something acidic to cut through: lemon, pickled onions, cornichons, or a sharp mustard. Plain bread rather than buttered. A small amount goes further than you expect, so pace the portions.

They are a strong second step, not a first one. Start with something in plain olive oil, learn what you like, then move to belly cuts and cod liver once the category is familiar.