• Preheat the oven to 350°F (175°C). Butter a flan pan or baking mold and set it aside.

Making the Caramel

• Pour the granulated sugar and the water into a heavy-based saucepan and set it over medium heat. Stir until the sugar has dissolved, then stop stirring and bring the mixture to a boil. Let it cook until the sugar has turned a rich golden brown, being careful not to let it burn.
• Immediately pour the hot caramel into the buttered flan pan and turn it so the base and sides of the pan are coated in caramel. Work quickly before it hardens!

Making the Quesillo

• Beat the eggs well, and then whisk in the condensed milk and vanilla until the mixture is combined. Beat in the whole milk and lemon zest.
• Pour the egg and milk mixture into the flan mold, and place in a large roasting pan. Create the bain marie by filling it with enough boiling water to go halfway up the side of the flan mold.
• Bake the quesillo at 350°F (175°C) for about 35-50 minutes, or until it has an internal temperature of 175°F (79°C). The baking time will depend on the pan you are using, so after about 35 minutes, check it frequently.
• Let the flan cool completely; preferably, chill it in the fridge overnight or for at least 4 hours.

Serving

• When you're ready to serve the quesillo, run a knife around the edge, then place a serving plate over the pan, flip it over, and carefully remove the pan. The flan should easily come out.
• Serve with some caramel sauce and enjoy!