In the old days in France, rillettes (essentially very tasty pâté-like potted meat) were always made from pork, duck, or goose, well-seasoned and rather fatty. Spread on a freshly baked baguette, they are still a fine thing. Now rillettes are concocted from almost anything imaginable, and why not? Fishy versions are frequently composed of salmon or shellfish mixed with butter. (However, a tuna salad sandwich I had on a recent Air France flight was called rillettes de thon, and this seemed a rather too glorified name.) I still dream about the mackerel rillettes served by a feisty young Parisian chef. She poached fresh mackerel in a flavorful white wine broth. At home, I use smoked mackerel fillets (or sometimes canned sardines).