The Jambon-Beurre Cornichons settles an argument that the plain version leaves open: where the pickle goes. The cornichon, the small tart gherkin packed in vinegar with tarragon and pearl onions, usually sits on the side of a jambon-beurre, a separate item you bite between mouthfuls. Here it goes inside, sliced lengthwise into thin strips and laid along the ham. That single decision changes the sandwich, and the people who order it this way do so deliberately.
The components are otherwise familiar: a baguette, a thick layer of barely-salted butter, slices of jambon de Paris cut to order. What the cornichon adds is a line of acid running the length of the sandwich, hitting in every bite rather than in the gaps between them. The vinegar cuts the butter's fat and lifts the gentle, faintly sweet ham, and the gherkin's snap plays against the soft crumb. The reason the pickle is more often kept outside is that it can wet the bread if it sits too long, so the cornichons go in at assembly and the sandwich gets eaten promptly, not packed for later.
The craft is in the slicing and the restraint. Cornichons cut too thick turn the sandwich sour and unbalanced; a few thin strips, blotted of their brine, are enough to register without taking over. The butter still has to be spread to the edges, the ham still has to be the good pale boiled kind, the baguette still has to have come out of the oven that morning. The pickle is a fourth voice, not a louder one, and the sandwich works when it stays in proportion to the other three.
Variations stay within the same logic. Some counters use cornichons aigres-doux, the sweet-sour style, which softens the acid. Others reach for thin-sliced pickled onion instead, or a smear of mustard alongside the cornichon for a sharper edge. Each is a small adjustment to where the acid sits and how loud it is. The base sandwich it adjusts is the Jambon-Beurre, and everything here is a reading of that sandwich with the pickle moved inside.