The fish finger and mushy peas sandwich is the one where the peas are structure before they are flavour. Mushy peas, marrowfat peas soaked and cooked down to a thick green pulp, are not a sauce dribbled on for seasoning here. They are a bed. Spread in a layer under the breaded fish, they do a job no liquid sauce can: they hold the fingers in place. The crumbed fish is the usual constant, a baton of white fish in a brittle orange coating, but where another sandwich relies on a stripe of sauce and a press to keep the row from sliding, this one sets the fingers into a soft, dense cushion that grips them. The defining decision is mechanical as much as it is to do with taste.
The craft is getting the pea layer right in both thickness and moisture. The peas have to be cooked stiff rather than loose, because a thin, wet mush will weep into the bread and turn the base to paste before the sandwich reaches a hand, while a stiff pulp sits as a cohesive bed and stays put. They are spread on one slice in an even layer thick enough to seat the fingers without burying them, since their job is to support and to add an earthy, faintly sweet counter to the fish, not to overwhelm it. The fingers go on hot and crisp straight onto the peas, and the soft white bread is buttered to the edges so the fat film still slows the moisture the peas inevitably carry. The top slice is pressed gently so the fingers settle into the bed and the whole thing binds, then it is eaten before the pulp has had time to soak through.
The variations move the same idea around. A dash of malt vinegar through the peas sharpens the earthy note toward the chip-shop register; mint cooked into them pulls it the other way; a stripe of tartare or ketchup added on top brings back a sauce the peas were already standing in for. The plain canonical fish finger sandwich and its cold-sauce readings are the relatives that leave the peas out entirely. Each deserves its own article rather than being crowded in here.