Bacon and mushroom is the bacon roll with a water problem built into it. Mushrooms are roughly nine-tenths water, and that single fact governs the whole sandwich. Adding fried mushrooms to bacon brings a deep, savoury, earthy note that pairs naturally with the cure, but it also introduces a wet, soft ingredient into a build whose entire structure depends on the bread not getting soaked. The defining question is not flavour, which takes care of itself, but moisture: a bacon and mushroom roll succeeds or fails on whether the mushrooms were dried out before they went anywhere near the bread. This is the variant where technique, not a condiment, is the difference between a good sandwich and a sodden one.
The build is therefore an evaporation exercise. The mushrooms have to be fried hard in a hot, fairly dry pan until they have given up their water and it has cooked off, going from pale and slack to browned and shrunken, because mushrooms added wet will weep into the bread within a minute and the sandwich collapses. The bacon is cooked until its fat has rendered and the edge firmed, partly for the salt-and-crisp counter against the soft mushrooms and partly because the bacon fat is a useful medium to finish the mushrooms in, browning them while seasoning them. The bread is chosen with the moisture load in mind: a soft roll works only if the mushrooms are genuinely dry, and a sturdier carrier is the safer choice when they are not. Butter on the cut faces seals the crumb as the last defence against whatever water the mushrooms still carry. Sauce is usually held back here, because brown or red on top of an already savoury, slightly wet filling pushes the whole thing past what the bread can hold.
The variations are the breakfast components this shares a pan with and the rest of the fry. Bacon and egg adds a yolk and another wet element to manage; bacon and tomato runs the same moisture problem with an acid filling instead of an earthy one; sausage joins the bacon for a heavier build. The brown-sauce and red-sauce camps are the plainer roll this elaborates on. Each of those deserves its own article rather than being crowded in here.