The primary ingredient in a Quiche Lorraine is eggs. This savory dish is a type of custard tart that traditionally includes cream, and it often features lardons or bacon along with cheese, typically Gruyère. The combination of these ingredients forms a rich, creamy filling that's baked in a pastry crust. While there are many variations, Quiche Lorraine remains distinct for its straightforward yet flavorful profile, making it a popular choice in French cuisine and brunch menus around the world.