Preheat the oven to 325°F (160°C). Prepare a 9-inch round flan mold or six 6-ounce ramekins and set them in a deep roasting pan for a water bath.
Make the caramel: in a small, heavy saucepan combine the Granulated Sugar with the Water and a splash of the Lemon Juice; heat over medium without stirring until the sugar melts and turns a deep amber color, swirling the pan occasionally. Remove from the heat and carefully pour the hot caramel into the bottom of your mold or divide among ramekins, tilting to coat the bottoms; set aside to harden.
Infuse the milk: in another saucepan warm the Whole Milk with the Lemon Peel and the Cinnamon Stick until it is steaming but not boiling. Remove from heat and let steep for 10–15 minutes so the flavors infuse, then discard the peel and stick.
Whisk the custard: in a large bowl gently whisk together the Eggs and the remaining granulated sugar until just combined (avoid whisking in too much air). Gradually pour the warm infused milk into the egg mixture in a slow stream, whisking constantly to temper the eggs.
Strain the custard through a fine-mesh sieve into a measuring jug or bowl to remove any bits of cooked egg and ensure a silky texture. Pour the strained custard over the set caramel in the mold or ramekins.
Place the filled mold(s) into the roasting pan and carefully pour hot water into the pan until it comes halfway up the sides of the mold(s) to form a bain-marie. Transfer to the oven and bake for 45–60 minutes, or until the custard is mostly set but still slightly jiggly in the center.
Remove the flan from the water bath and let cool to room temperature, then refrigerate for at least 4 hours or overnight to fully set and develop flavor.
To serve, run a thin knife around the edge of the mold, invert onto a serving plate, and lift off the mold so the caramel sauce flows over the custard. Slice and enjoy chilled.
