Add sugar to a saucepan and cook over medium heat until melted and golden brown.
Pour the caramel into a baking dish or individual ramekins and spread evenly.
Whisk eggs, milk, condensed milk, lemon juice, lemon zest, and vanilla extract until smooth.
Strain the custard mixture to create a silky texture.
Pour the lemon custard gently over the caramel.
Place the dish in a water bath and bake until the flan is set but slightly jiggly in the center.
Cool completely, refrigerate for several hours, then invert onto a serving plate and garnish with fresh lemon zest.