Of every filling in the family, uovo e maionese is the one stripped down to two ingredients and a slice of bread, and that is exactly its argument. There is hard-cooked egg and there is mayonnaise, and nothing is asked to do a job the other two cannot. The egg, chopped, brings the body, the gentle richness, the faint sulfur. The mayonnaise brings the fat that binds the egg into a smooth cohesive mass and seals the soft crumb against it. The bread is soft white pancarrè with the crust gone, a pale tender frame that adds structure and a mild backdrop and nothing louder. With no anchovy, no tuna, no caper, no leaf, the build has nowhere to hide: the egg has to be cooked right and the bind judged exactly, because there is no second flavour to cover a fault. Take the mayonnaise away and the egg is dry rubble; take the egg away and the mayonnaise is a smear of fat. The two are arranged to need each other and almost nothing else.
A good one is plain food done with care. The bread is genuinely fresh, soft enough to give under a thumb, trimmed clean of every edge of crust so only the pillowy crumb remains, and held under a damp cloth so the cut faces stay supple. The eggs are cooked through but not chalky, the yolks fully set and still tender with no grey ring, then chopped fine enough to bind but coarse enough to keep a little texture in the bite. The mayonnaise goes in by feel, enough to make the egg cohere into a spreadable mass that does not slide, often with a small pinch of salt, and that is the whole seasoning. The same bind films the inner faces of the bread, a thin barrier coat that keeps the crumb soft against the filling's moisture in the window before eating. The egg is mounded toward the centre so the triangle domes, fullest in the middle and thin at the cut. A sloppy build uses overcooked grey-ringed eggs drowned in too much mayonnaise that soaks through; a careful one cooks the egg right, binds just enough, centres the dome, and cuts a pale clean diagonal.
The close cousins each deserve their own article rather than being crowded in here. Stir anchovy through the mix and the salt and the sea arrive, a different build entirely. Fold tuna into the egg and the filling turns marine and denser. Add a thread of mustard and the lift sharpens past the plain version. Lay tomato or lettuce against the egg and the moisture and crunch shift the whole math. The uovo e maionese is the family reduced to its barest two parts on soft bread, and every other egg build is best read against this one first.