Which pastry is used for fruit flans?

None. A properly made flan contains no pastry. The ingredients are eggs, sugar, cream, and flavoring. To prepare the flan, caramelized sugar is poured into the bottom of a glass or ceramic baking dish. A custard is made from mixing the eggs, cream, and flavoring, along with a little more sugar. The custard mixture is poured over the caramelized sugar in the baking dish, and the whole thing is put in a slow oven to bake. Overbaking coagulates the eggs and turns the flan into a grainy mess, so it needs to be monitored carefully. It's done when a cake tester comes out clean. To serve, cool, loosen the sides if necessary with a knife, and flip the baking dish over onto a plate. The flan may be accompanied by whipped cream and/or fruit.

Some common flavorings include orange, lemon, or coconut. I've also had chocolate flan, which contains cocoa powder or melted chocolate in the custard mixture. Delicious!

If the flan you had seems to have a pastry crust, either the chef deviated from the standard recipe a bit and added a crust, or the flan was baked too long and the eggs crusted over. I recently had a flan in a Mexican restaurant that added cinnamon graham cracker crumbs as a crust. Good, but not standard.