The defining problem of a beetroot sandwich is not flavour but bleed. Sliced pickled beetroot is earthy, sweet, and sharp with vinegar, a genuinely good filling, but it carries a deep crimson liquid that migrates into white bread on contact and turns the whole slice pink within minutes. Every real decision in building one is a decision about containment. The sandwich is not difficult because beetroot is hard to like; it is difficult because beetroot will not stay where it is put, and the craft is entirely about managing a filling that stains.
The work is drainage and a barrier. The slices are lifted from the jar and left to drain properly, because the vinegar liquor is what does the bleeding and most of it can simply be removed before assembly. A layer of butter spread to the edges, or a smear of cream cheese or salad cream, waterproofs the crumb and holds the colour off long enough for the sandwich to be eaten rather than admired. The bread is soft and plain so the beetroot's earthy sweetness reads clearly, and the slices are arranged in a single even layer rather than stacked, because a stack concentrates the bleed at one point and floods through. This is a cold sandwich with no heat to manage; its entire success rests on whether the maker respected the liquid. Done with care it tastes like a pickled garden; done carelessly it is a wet pink mess by the time it reaches the mouth.
Beetroot rarely travels alone for long, and what joins it is usually a salt counter to its sweetness. Cheese is the standard answer and a distinct sandwich in its own right; egg, cold beef, and a sharp Cheddar each set saltiness against the earthy note while inheriting the same staining problem. Those deserve their own articles rather than being crowded in here.