Add the sugar and a little water to a pan. Heat on the lowest setting so the sugar slowly melts. Once it turns liquid and golden, stir constantly, squeeze in the lemon (to keep the color and add aroma), then immediately turn off the heat! Pour a thin layer of caramel (about 1 to 1 1/2 teaspoons, or more depending on mold size) into the bottom of each mold or container.

Mix the whole milk (using unsweetened milk makes it richer) and sweetened condensed milk together, stirring gently. Don’t whisk vigorously to avoid bubbles. Taste and add more sugar if you like it sweeter.

Heat the milk mixture over medium heat. When it just starts to simmer, turn off the heat—don’t let it boil hard or it will lose quality. Crack the eggs into a bowl. Gently beat the eggs in one direction, being careful not to create bubbles (bubbles will make the flan have holes). Once the eggs are well mixed, strain them through a sieve 2-3 times to make the mixture smooth and remove any stringy whites. Slowly pour the warm milk into the eggs while stirring gently in one direction (don’t pour the eggs into the milk, or the eggs will cook and the flan will be lumpy). Strain the mixture one more time.