Heat milk, over low heat, in a small saucepan on the back burner and keep warm until ready to use.
In a bowl, whisk together egg yolks, sugar, and cornstarch; slowly whisk in a little of the hot milk to temper the eggs.
Pour egg mixture into warm milk and stir constantly until thickened a bit (flan will thicken while it is chilling in the refrigerator).
While the custard cools, peel oranges and divide them into sections: place into a serving bowl or mold; sprinkle oranges liberally with brown sugar (I stir and crush them a little to release some of the juice).
Pour cooled custard over oranges; sprinkle with cinnamon.
Chill, in the refrigerator at least 2 hours, loosen carefully from sides, invert onto a plate, and serve.