Flan is a type of CUSTARD-based dessert that can be served in a variety of forms. It is typically made with eggs, milk, sugar, and vanilla, and can be baked or unbaked. The most common form of flan is a TART, which is a PASTRY shell filled with a CUSTARD filling. QUICHE is another form of flan, which is a savory TART made with a CUSTARD filling and various other ingredients. Flan can also be served as a PASTRY, which is a dough-based dish filled with a CUSTARD filling. Finally, flan can also be served as a PIE, which is a PASTRY crust filled with a CUSTARD filling.