Hey MAO,
Flour and Sugar!
Both are prepared in a custard like manner. They will both have eggs and milk.
Quiche has the addition of Flour (and anything else you want to add)
Flan has the addition of Sugar (As a Caramel) in the bottom of the custard. When baked, the sugar caramel liquefies and that is the “Sauce” that tops the flan.
And as you know, BOTH ARE REALLY GOOD!
Good Eating,
James in San Diego