The name "Flan" is begins with a word in old French "flaon", which comes from Latin "flado" meaning "custard". According to Alan Davidson, Oxford Food Companion " The same Latin root was used in the Middle English word "flaton", and "flawn" which later became "flan".
The roots of Flan, and really custards in general date back to Medieval times. This eggy course is mentioned in both Ancient Roman and Medieval European food history. Most likely early flans were savory but we have see recipes for sweet versions too. There is a recipe for a Honey custard in "A Taste of Rome" by Ilaria Gozzini Giacosa (pg 162) certainly a flan-like preparation.
Eggs were thought to have many health benefits (and aphrodisiac qualities as well). These custard dishes were spread through Europe with the Roman conquerors. " According to Platina's De Honesta Voluptate[On Right Pleasure and Good Health], an Italian cookery text published approximately 1475, custard-type dishes were considered health food. In addition to being nourishing they were thought to soothe the chest, aid the kidneys and liver, increase fertility and eliminate certain urinary tract problems."
"[English] Roman period...eggs took on a much greater importance in Roman times, when domestic fowl first became common. With eggs for the first time available on such a scale, it was now possible to consider them seriously in cookery..[the Romans] exploited eggs as a thickening or binding agent for other foods. They borrowed from the Greeks the idea of combining eggs with milk to form a custard mixture, which was either cooked very slowly in an earthenware pot, or fried in oil...Another kind of egg confection was made of fruit or vegetables, or fish or shredded meat, bound with eggs and lightly cooked in the open dish called a "patina." ...The "flathons" (flans), "crustards" and other open tarts of medieval cookery again recall the old "patinae," with the shallow open dish of the Romans replaced by an open pastry crust, and the filling once more mixed and bound with eggs." ---Food and Drink in Britain: From the Stone Age to the 19th Century, C. Anne Wilson (p. 138,p.142) FoodTime
The Flan pastry is baked in a Flan ring (see picture) atop a baking sheet. Flan may also be baked in a tart pan or a pan with a removable bottom. A filling is added to the baked pastry. Fillings may be of any type but typically they are custard with a fruit topping or cheese custard resembling a Quiche.
Flan Ring - for preparing the "tart" style flan
In many countries such as Mexico, Spain, as well as Cuba it is customary to make flan in a special pan (mold) over a bain marie (water bath). The molds are fitted with a lid that clips on securely. The custard can be prepared on the cook-top or in the oven. First you add sugar and water to the pan and swirl over a hot burner to melt the sugar and form the caramel. The pan is then filled with the custard mixture and placed over a pot of boiling water (either in an oven or over a double boiler) where it remains until the custard is set, about 1 hour.
The custard is cooled, then inverted on a plate. The flan is cut in wedges and served with the caramel poured over the top.
flan pan with double boiler and lid