"Mixed greens regicide"?
It clearly means that if you are the true heir to the throne of France and you consume this salad, it will kill you. It's sort of a sword-in-the-stone kind of thing, except instead of a stone, there's mesclun lettuce.
Obviously, as tradition attributes to the king the power to heal with the touch of his hand, the "Leech Salad" is a reference to this form of traditional healing. The "poison" of modern republicanism will be "leeched" from the state by the arrival of the true heir to the throne, who will be recognized, hailed, and then, alas, keel over into the vinaigrette.
I think this menu was composed by a senecan monarchist who wished to comment on the recent elections in France. Or by someone with a very high fever . . .