Vert-Pré sauce (sometimes vert pres) is a green herb sauce made with spinach, parsley, chives, tarragon, and chervil in a base of shallots, butter, and velouté. Larousse 1961 also uses the term to refer to green mayonnaise.
In French haute cuisine, to prepare a grilled meat au vert-pré is to garnish it with straw potatoes an watercress, and to serve with maître d'hôtel butter containing a generous amount of parsley. (Larousse 1961)