Egg and tomato is the bound-egg sandwich with a problem built into it, and the problem is water. The constant is the same chopped egg held with mayonnaise; the variable is sliced tomato, which brings exactly the acidity a rich, mild egg filling wants and, in the same slice, a flood of juice and seeds that runs straight into soft bread and a soft filling. The defining fact of this sandwich is therefore not flavour but the bleed: a good egg and tomato is balanced and bright, a bad one is a wet, collapsing thing eaten over a plate, and the difference between the two is entirely whether the tomato's water was controlled before assembly.
The craft is keeping the tomato from destroying the build. The slices are cut a sensible thickness, the wet seed pulp can be scooped out, and the cut faces are salted and left to drain and then patted dry, because every drop left in them ends up in the crumb within the hour. The bound egg is made on the firm side, held with just enough mayonnaise to cohere, and the butter is taken hard to the edges of the bread to lay a waterproof barrier between the tomato and the crumb. Order matters: the egg goes against the bread and the drained tomato sits inside the egg rather than directly on a slice, so the filling itself takes the moisture instead of the bread. Salt and white pepper go through the egg in the bowl. It is built close to when it is eaten, because even a well-drained tomato gives this sandwich a short life.
The variations are the other bound-egg pairings, separated by what they set against the egg and what trouble that thing brings with it. Cress and watercress add pepper and crunch with no water penalty; chive adds an allium edge; salad cream changes the binder rather than the addition. Each deserves its own article rather than being crowded in here.