Anchovy turns the mild egg tramezzino into something with a deep salty undertow, and the tramezzino uovo e acciughe is built around the tension between the two. The egg is hard-cooked and chopped, soft and rich and faintly sulfurous, bound with mayonnaise into a pale spreadable mass. The anchovy is the cured fillet or its paste, intensely savoury and salt-cured, and it does not sit on top of the egg so much as run underneath everything as a low, marine, umami current. The egg gives the body and the calm; the anchovy gives the depth and the salt the egg cannot reach on its own. The mayonnaise binds the egg to itself and the whole to the bread and seals the crumb, while the soft white frame mutes the anchovy so it reads as savoury rather than aggressive. Take the anchovy out and this is the plainest egg build; take the egg out and the anchovy is a salt slick with nothing to carry it. The two are arranged to need each other across the line where mild richness meets cured intensity.
A good one is a lesson in dosing the anchovy. The bread is soft white pancarrè, fresh that day, soft to a thumb, all crust trimmed so only the tender crumb remains. The eggs are cooked through but not chalky, yolks set and still tender with no grey ring, chopped fine enough to bind and coarse enough to keep a little texture. The anchovy is worked in small, mashed into the mayonnaise or laid as thin fillets and chopped through, never piled, because a heavy hand turns the whole sandwich into salt and nothing else. Enough mayonnaise goes in to make the egg cohere into a spreadable mass without sliding, the salt judged against the anchovy already present. The bind also films the inner faces of the bread so the crumb stays dry before eating. The mass is mounded toward the centre so the triangle domes, fullest in the middle and clean at the cut. A sloppy build over-anchovies one corner and leaves the rest plain; a careful one spreads the cure evenly through the egg, keeps the salt in check, centres the dome, and cuts a diagonal that holds.
The close cousins each deserve their own article rather than being crowded in here. Swap the anchovy for plain mayonnaise and you have the simplest egg tramezzino, calm and unsalted by cure. Fold tuna into the egg instead and the marine note comes from flaked fish rather than a cured slick. Stir capers through alongside and the salt sharpens to a brighter point. Lay the egg with tomato or lettuce and the moisture and crunch change the build entirely. The uovo e acciughe is the egg base pulled toward the sea by a small salty hand, and it is best read against the plain egg version first.