Bacon and tomato is the bacon roll that builds its acid in instead of bottling it. The sauce camps reach for brown or red to cut the cure; this version puts a sliced or grilled tomato into the sandwich to do the same job with fruit acid and water rather than vinegar and sugar. That is the defining trade. A tomato against salt-cured bacon supplies a bright, slightly sweet, distinctly fresh counter that a sauce cannot, the acid lifting the fat and the cure, but it does it with a soft, wet ingredient instead of a controlled stripe of condiment. The interest of this variant is that it solves the bacon roll's balance problem with produce, and inherits a moisture problem in exchange.
The build splits on whether the tomato is fresh or grilled, and each route has a different failure. Fresh tomato brings the cleanest acid and the most water, so it is sliced and ideally salted and drained, or at least kept off the bare bread by the bacon, because raw tomato laid straight on the crumb weeps through fast. Grilled tomato concentrates as it cooks, the heat driving off water and intensifying the sweetness, which trades some freshness for a filling that behaves better in bread. Either way the bacon is cooked until the fat has rendered and the edge crisped, both for the salt-and-crunch counter to a soft, juicy tomato and so the rasher holds its structure against the moisture. Butter on the cut faces seals the crumb against tomato water, and the bread is chosen for the version: a soft roll copes with grilled tomato that has been cooked down, while fresh slices want either draining or a sturdier carrier. Most builds skip the bottled sauce entirely, because the tomato is already supplying the acid and adding ketchup on top is the same note twice.
The variations are the rest of the fry-up this draws from and the sauce roll it reworks. Bacon, lettuce, and tomato stacks the same idea toward a club logic; bacon and egg adds a yolk to the acid; bacon and mushroom swaps the wet filling from acid to earthy. The brown-sauce and red-sauce camps are the bottled version of the job the tomato is doing here. Each of those deserves its own article rather than being crowded in here.