The Pa amb Tomàquet Garni starts not with a filling but with a treatment of the bread itself. Pa amb tomàquet, Catalan for "bread with tomato," is bread rubbed with a halved ripe tomato until the pulp and seeds work into the crust, then dressed with olive oil and salt. In Roussillon, the Catalan-speaking corner of southern France where this belongs, that tomato-rubbed bread is a base in its own right, and garni means it has been built up into a closed sandwich rather than left open. The defining element is the bread preparation: the tomato is not a slice laid on top, it is worked into the loaf before anything else goes near it.
The build is a country loaf or a length of rustic bread, split and rubbed hard with cut tomato so the surface turns red and slightly damp, then drizzled with olive oil and salted. Onto that goes the filling, most often a dry-cured ham or a regional cheese, sometimes both. The logic is that the bread is already seasoned and already carrying moisture and fruit acidity, so the filling can be spare. The tomato-rubbed surface does the work that butter does in a northern sandwich: it carries fat from the oil, brightens with acid, and keeps the crumb from being dry, which is why a heavy spread is unnecessary here.
The craft is in the tomato and the timing. The tomato has to be ripe enough to break down into the bread under pressure; an underripe one just scrapes across the crust without giving anything up. Too much pulp and the bread turns soggy before it reaches the table, so the rub is firm but brief and the sandwich is assembled close to when it is eaten. A good crust matters more than usual, because it has to hold the tomato and oil without collapsing.
Variations follow what gets laid on the seasoned bread: a salt-cured ham, a sheep's-milk cheese, a few of both, occasionally an anchovy for a sharper, brinier reading along the coast. Each is a different filling on the same tomato-rubbed base. The wider tradition of bread carrying oil and time until it becomes a sandwich is the Pan Bagnat, to which this Catalan-rooted version is a close southern relative.