Beurre blanc is a French butter sauce made by reducing white wine or vinegar with shallots, then whisking in cold, cubed butter until it emulsifies into a smooth, velvety sauce. It comes from the Loire Valley near Nantes, where it's traditionally credited to a chance mistake -- a cook, often named as Clémence Lefeuvre, said to have forgotten the egg yolks while making a sauce for pike around the turn of the 20th century.
High-quality butter matters most here, since it's most of the sauce; shallots and acid balance its richness. Beurre blanc is classically served with poached or grilled fish, scallops, and lobster, though it also works with poultry, vegetables, and roasted meats. Fresh herbs like chives, parsley, or tarragon, plus additions like lemon zest, garlic, or truffle, are common variations.