Preheat oven to 350 degrees. Spray a 9 inch cake pan with cooking spray. You will need a pan larger than your cake pan to make a water bath.
In a medium sauce saucepan over medium-low heat, melt sugar. Continually stir until sugar liquifies and turns golden caramel. Pour into 9 inch cake pan. Tilt pan to coat the bottom with caramel. Set aside.
Whisk eggs in large mixing bowl until completely incorporated. Making sure white and yoke and combined. (This helps make the flan smooth and cook evenly.) Whisk in milks. Then add lemon zest and juice.
Pour egg mixture on top of caramel. Set cake pan inside larger pan. I pull out my oven rack, set the pan on it and then fill the water bath half way up the flan pan.
Bake for 1 hour. Let cool completely. Can be served at room temperature or refrigerated and served cold. When ready to serve run a knife around the edge to loosen flan. Then invert on to a serving plate.