The panino con tartare di manzo is governed by one rule that overrides every other choice: the beef is raw, so freshness is the whole sandwich. Hand-cut or finely chopped lean beef, seasoned simply with oil, salt, and often a little lemon, is laid on bread without ever touching heat. Because nothing is cooked to mask anything, the quality and freshness of the meat is not one factor among several, it is the entire proposition. This is the logic of the gourmet paninoteca: not stacking more in, but sourcing one premium thing well and removing everything that would interfere with it. The bread is a frame and a handle for raw beef at its cleanest, nothing more.
The craft is the cut, the seasoning, and the timing. The beef is trimmed of all sinew and cut by knife rather than ground, so it stays in distinct tender pieces with a clean bite instead of a paste; it is dressed at the last possible moment, since salt and lemon left on raw meat begin to cure and grey it within minutes and the point is the bright red freshness. The bread is sturdy but mild and often lightly toasted for structure, never assertive, because a strong crust or a vinegared element would overwhelm a delicate raw flavour that has no cooking to stand behind it. The counter, when there is one, is restrained and chosen to lift rather than to compete: a few drops of good oil, a turn of pepper, a shaving of Parmigiano or a single anchovy worked into the seasoning. Temperature matters as much as anything; the meat is served cool, not cold and not warming, the window in which raw beef tastes cleanest.
The named variations are about what single accent is allowed near the raw beef. There is the version finished with shaved truffle, the one bound alla piemontese with oil and lemon and little else, and the build with a raw egg yolk folded through. The seared and cooked beef panini are a different idea entirely and deserve their own articles rather than being crowded in here.