Verjus is the pressed juice of grapes picked before they ripen — sharp, sour, but without vinegar’s sharp volatile edge. Kitchens used it constantly before citrus and wine vinegar became cheap and available year-round; we brought it back because it does something neither of those substitutes quite does: it cuts richness without announcing itself.
It shows up most obviously in the duck course, but a small bottle of it sits on nearly every station in the kitchen — the acid of choice whenever a dish needs brightening without wanting to taste sour.