Prepare the pan(s): Use a 1-quart (1-liter) round baking dish or 6 individual ramekins. Place them in a deep roasting pan to set up the water bath later.

Make the caramel: In a small saucepan, combine sugar, water, and lemon juice. Cook over medium heat without stirring until the syrup turns a deep amber, 7–10 minutes. Swirl the pan gently if needed.

Coat the dish: Immediately pour the hot caramel into your baking dish or divide among ramekins. Tilt to coat the bottom evenly. Set aside to cool and harden.

Warm the milk: Heat the milk over medium-low until steaming but not boiling. Remove from heat. Stir in vanilla and a pinch of salt.

Mix the custard: In a bowl, whisk eggs, yolks, and sugar until just blended. Do not whip air into it. Slowly pour in the warm milk while whisking gently.

Strain for smoothness: Pour the custard through a fine-mesh strainer into a pitcher or large measuring cup. This removes bubbles and bits for a silky texture.

Fill the pan: Pour the custard over the set caramel.

Set up the water bath: Place the roasting pan with the filled dish on the oven rack. Carefully pour hot water into the roasting pan until it reaches halfway up the sides of the baking dish.

Bake: Bake at 325°F (165°C) for 35–45 minutes for ramekins or 50–65 minutes for a large dish. The flan should be set around the edges with a slight wobble in the center.

Cool and chill: Remove from the water bath. Let cool to room temperature, then cover and chill at least 4 hours, preferably overnight.

Unmold: Run a thin knife around the edge. Invert a rimmed plate over the dish, flip, and lift off. The caramel will cascade over the flan.