The panino con testa in cassetta is built around a set, sliceable terrine rather than a soft filling. Testa in cassetta, the Ligurian head cheese, is made by simmering the cuts of a pig's head until the meat falls and the liquor turns rich with natural gelatine, then pressing everything into a mould, often with pistachios or peppercorns, so it cools into a firm loaf bound by its own jelly. Sliced thin onto bread, it reads as a mosaic of tender meat suspended in a clean, faintly wobbly aspic. That gelatinous set is what defines it: it is not a cured slab and not a spread but a terrine that holds its shape on the bread and yields softly under the teeth.
The craft is the slice and the temperature. The terrine is cut thin, because a thick slab is heavy with jelly and reads as cold and dense, while a fine slice lets the meat and the set jelly come apart cleanly in the mouth. It is served cool but never fridge-hard, since real cold turns the gelatine rubbery and flattens the gentle, mildly spiced flavour; brought to cool room temperature the jelly softens and the seasoning, the pepper and pistachio worked into the loaf, comes forward. The bread is plain and sturdy, a crusted roll, doing structural work only, because the terrine already carries its own salt, fat, and aromatic spicing and a strong bread would simply argue with it. The classic counter is sharp and minimal: a few cornichons, a smear of mustard, a leaf of something bitter, just enough acid to cut a rich, jellied meat without burying it.
The named variations stay within the head-cheese family and its region. There is the build with sottaceti, the version with mustard, and the related pressed-meat terrines such as coppa di testa and soppressata in its set Tuscan sense. Each is its own preparation and deserves its own article rather than being crowded in here.