To make the caramel: have ready a 2-quart round flan mold and a large roasting pan. Combine 1 cup of the sugar and 2 tablespoons of water in a heavy-bottomed pot or pan. Place over medium-high heat and cook until the sugar begins to melt. Swirl the pan over the heat until the syrup darkens to a medium amber color, about 10 minutes; don't stir with a spoon. Remove from the heat and immediately add the lemon juice, swirl the pan again to combine, and then pour into the flan mold. Tilt the dish so that the caramel evenly coats the bottom and a bit up the sides, place in the roasting pan and set aside.
Preheat the oven to 325 degrees F, bring a kettle of water to a boil for the water bath and keep it hot.
Combine the cream, cinnamon, and vanilla in a small saucepan over medium-low flame. Bring the cream to a brief simmer, stirring occasionally. Take care not to let the cream come to a full boil to prevent it from spilling over.
In a large bowl, cream together the whole eggs and yolks with the remaining 1/2 cup of sugar; add a pinch of salt. Whisk until the mixture is pale yellow and thick. Temper the egg mixture by gradually whisking in the hot cream mixture; don't add it too quickly or the eggs will cook. Pass the mixture through a strainer into a large measuring cup to ensure that the flan will be perfectly smooth. Pour the custard into the caramel-coated mold.
To create the water bath: pour the hot (not boiling) water into the roasting pan to come halfway up the side of the mold; be careful not get water into the custard. Carefully transfer to the middle oven rack, and bake for 30 to 45 minutes, until the custard is barely set and just jiggles slightly. Let the flan cool in the water bath, then refrigerate for at least 4 hours or overnight.
When you are ready to serve, run a knife around the inside of the mold to loosen the flan. Place a dessert plate on top of the flan and invert to pop it out.