Combine 1/2 cup sugar (100 grams) and 1/3 cup water (100 ml) in a saucepan. Stir well, then cook over low heat until the sugar turns a deep amber color. Add the lemon juice, gently swirl the pan to mix (do not stir with a spoon or chopsticks). Turn off the heat and quickly pour the caramel into each flan mold (enough for 21 molds).

In a bowl, add 10 egg yolks and 6 egg whites. Add 1/4 cup sugar (50 grams) and gently whisk until the sugar dissolves.

Pour the milk into a saucepan and heat over low heat until it just starts to steam (do not let it boil). Gradually add the warm milk to the egg mixture, whisking gently as you pour.

Strain the egg-milk mixture through a fine mesh sieve. Pour the mixture evenly into each flan mold.

Gently tap each mold and use a toothpick to pop any bubbles on the surface. Let the molds sit for 10–15 minutes before steaming (this helps the tops turn a nice golden color).

Line the bottom of the steamer with a kitchen towel. Arrange the molds inside (do not cover the molds). Bring a pot of water to a boil, then place the steamer with the molds over the pot. Steam over medium heat. Every 5 minutes, open the lid and wipe off any condensation. Steam for about 15–20 minutes, or until a toothpick inserted in the center comes out clean. Note: Do not steam over high heat or the flan will become porous. Remove the flan from the steamer and let cool. Store in the refrigerator for up to 5 days.