The casual watercress sandwich is the same peppery leaf as the tea-tray finger with the ceremony removed, and the missing ceremony is what defines it. The crusts stay on. The cress goes in by the handful rather than picked leaf by leaf, stalks and all. It is built to be eaten standing at a worktop or handed across a kitchen, not arranged on a tiered stand, and that change of setting changes the sandwich. This is watercress as an everyday filling rather than a delicacy: brisk, peppery, plain, made because there is a bunch of cress and a loaf in the house and that is enough for lunch.
The craft is still the leaf's pepper against bread and butter, but the tolerances are looser and that is the point. Watercress holds water, so it still wants washing and a reasonable shake dry, but a casual build forgives a few coarse stems left in and a quick rough dry where the formal finger does not. Butter is spread to the edges to seal the crumb and supply the only seasoning the sandwich gets, and it earns its place here as the salt that lifts the mustardy heat of the cress. The bread is soft white or a plain wholemeal, sliced normally and left whole with its crust, because the crust is no longer treated as an enemy of a delicate filling: this filling is not being protected, it is just being eaten, and a sturdier slice actually suits a generous, leafy load better. It still wants making close to when it is served, since watercress wilts on a wait whatever the occasion.
The variations are the everyday additions a tea sandwich would refuse on principle. A grind of black pepper or a smear of salad cream for a sharper edge; a slice of cheese or ham turning it from a light thing into a proper lunch; egg mayonnaise bulked through it for the working version of the egg-and-cress pairing. The crustless, leaf-picked, finger-cut version is a different sandwich with a different brief, and that one along with each of these deserves its own article rather than being crowded in here.