The panino con acciughe al verde is a Piedmontese sandwich built on a single relationship: a fiercely salty preserved fish cut by a sharp green sauce. The anchovies are salt-cured, filleted and rinsed, intense and oily and far too loud to eat in quantity on their own. The salsa verde is the Piedmontese bagnèt verd, parsley pounded with garlic, capers, a little soaked bread or egg, vinegar, and oil into a bright, acidic, herbaceous paste. Put the two on bread and the sauce does the entire job of making the fish edible at a sandwich's scale, lifting and cutting the salt rather than masking it. That balance is the whole point; the bread is mostly there to carry it and to give the mouth a neutral floor.
The craft is proportion and the order of assembly. The anchovy is the strongest voice and is used in the smallest measure, a few fillets laid flat rather than piled, because the sandwich fails the moment the fish overruns the sauce. The bagnèt verd is spread on both faces of the bread, generously, so its acid and the green sharpness meet the fish from both sides and no bite is only salt. The bread is plain and not too crusted, a soft roll or a slice of country loaf, since the sandwich is about the fish-and-sauce axis and an assertive bread would only crowd it. Nothing is heated: this is a cold assembly where the cure has already done all the cooking and the sauce is doing all the seasoning.
The variations stay Piedmontese and stay on the same axis: the version that adds a roasted pepper or a sweet peperone to answer the salt with sweetness, the one built with butter under the anchovy in the northern manner, the bagna càuda register where the anchovy is warmed in oil and garlic instead of dressed cold. Each of those is a different Piedmontese anchovy preparation, and each deserves its own article rather than being crowded in here.