The tuna mayo sandwich is built in the bowl, not the bread. Tinned tuna is drained and folded with mayonnaise until it just binds, seasoned, and often given a crunch, sweetcorn, cucumber, or onion, before it ever meets a slice. The bread is a passive carrier; everything that determines whether the sandwich is good has already happened in the mixing. That is the defining fact of the form and the reason it is both ubiquitous and so often mediocre.
The craft is moisture and contrast. Too much mayonnaise and the filling slides and soaks the bread; too little and it is dry and dull. The fix is a measured bind plus a hard textural counter: sweetcorn for pop, cucumber for water-crisp, red onion for bite. The bread is soft and plain and the filling is spread evenly so each bite is the same, because an uneven tuna sandwich is mostly bread at one end and a wet clump at the other.
The variations are small and honest. Tuna crunch leans on the vegetable counter; tuna and sweetcorn is the lunchbox default; the tuna niçoise borrows egg, olive, and bean from the salad. Each deserves its own article rather than being crowded in here.