In a small saucepan, combine the sugar, water, and lemon juice. Heat over medium heat without stirring until the mixture turns a deep amber color, approximately 3–5 minutes. Remove from heat and briefly place the saucepan in an ice bath for about 10 seconds to stop further cooking. Carefully pour the caramel into individual molds and allow it to harden.

In a medium bowl, gently whisk the eggs until combined. Set aside.

In a separate saucepan, combine the milk, sugar, vanilla extract, and salt. Heat over medium heat, stirring continuously until the sugar is fully dissolved and the mixture reaches approximately 70°C / 158°F. Do not allow it to boil.

Remove the milk mixture from heat. Slowly pour it into the eggs through a fine strainer while gently whisking to prevent curdling. Strain the mixture once more for a smooth custard.

Pour the custard mixture into the caramel-lined molds. Gently tap the molds on a folded towel to release any air bubbles. Cover each mold with aluminum foil and poke a few small holes in the foil.

Preheat the oven to 150°C / 300°F. Place a folded kitchen towel in a baking tray, arrange the molds on top, and carefully pour warm (not boiling) water into the tray until it reaches halfway up the sides of the molds.

Bake for 35 minutes, or until the custard is set but slightly jiggly in the center.

Remove molds from the water bath and allow to cool at room temperature for 10 minutes. Once cool enough to handle, remove the foil, cover with plastic wrap, and refrigerate for at least 4 hours or preferably overnight.

To unmold, run a knife around the edge of each flan and invert onto a serving plate, allowing the caramel to flow over the custard.