Preheat the oven to 325°F.

Carefully place the sugar in the center of a sauce pan. Pour the water and corn syrup over the sugar, making sure not to get sugar on the sides of the pan (see note). Cook over high heat and do not touch until the sugar starts to caramelize and turn an amber color. 

Once it reaches a dark amber color, remove from heat and pour into a flan mold, 9" glass pie dish, or individual ceramic ramekins, carefully moving the vessel to coat the bottom and sides.

Beat the eggs and add the milk. Stir in the condensed milk, zest, vanilla, and salt. 

Put the flan pan in a roasting pan then add the custard.

Add boiling water to the roasting pan to half way up the flan (see note).

Bake at 325°F for 40 minutes, checking the doneness by jiggling the pan. If it moves like set jello and an inserted knife comes out clean, it's done!

Let the flan cool in water bath to room temperature and refrigerate for 2 hours. After chilled, run knife around the edge and flip onto a plate!