The tomato sandwich is a sandwich that depends entirely on one ingredient being at its best. There is no protein, no second flavour, and nothing to distract from the fruit: thick slices of ripe tomato on soft buttered bread with a firm shake of salt, and that is the whole build. The salt is not a seasoning detail, it is half the sandwich. It draws the juice to the surface of the cut tomato and turns it savoury, so what is between the bread is sliced fruit carrying its own salted liquor. Made with a hard, pale, out-of-season tomato it is a sad thing; made with a ripe one in summer it is the plainest good sandwich there is.
The craft is almost entirely about controlling water, because a sliced tomato is mostly water and it leaks. The tomato is sliced thick rather than thin so it holds its shape and bleeds slowly, and it is often salted and left to sit for a moment so the first rush of juice runs off before the slices ever touch the bread. The butter is structural here, spread to the very edges so it waterproofs the crumb and slows the moment when the bread goes from soft to soaked. Even done well this is a sandwich with a short window: it is built to be eaten soon after it is made, because the same juice that makes it taste of summer is the juice that will, given time, turn the bottom slice to pulp. Plain soft bread is the right bread precisely because it yields to a soft filling instead of fighting it.
The variations stay close to the fruit and add one sharp or savoury note. A grind of black pepper, a few torn basil leaves, a thin smear of salad cream or mayonnaise instead of butter, a little raw onion, a slice of cheese to make it a meal: each keeps the tomato in the lead and lets one thing answer it. The crustless afternoon-tea tomato finger is the formal cousin and is treated on its own. Each deserves its own article rather than being crowded in here.