01 - Preheat the oven to 350°F.
02 - In a heavy saucepan, combine the granulated sugar and water. Cook over medium heat without stirring until the sugar dissolves and turns a golden amber color, approximately 7 to 9 minutes. Immediately pour the hot caramel into a 9-inch round baking dish or flan mold, tilting to evenly coat the bottom. Set aside to let it harden.
03 - In a large mixing bowl, whisk together the eggs, egg yolks, and salt until well combined. Add the sweetened condensed milk, evaporated milk, whole milk, vanilla extract, and orange zest if using. Whisk until the mixture is smooth and homogeneous.
04 - Pour the custard mixture through a fine-mesh sieve into the caramel-lined baking dish to ensure a silky, smooth texture.
05 - Place the flan dish inside a larger roasting pan. Fill the roasting pan with hot water to reach halfway up the sides of the flan dish, creating a water bath for gentle, even cooking.
06 - Bake for 60 minutes, or until the center barely jiggles when gently tapped.
07 - Remove the flan from the water bath and allow it to cool to room temperature. Refrigerate for at least 3 hours or until thoroughly chilled.
08 - Run a thin knife around the edge of the dish to loosen the flan. Invert a large serving plate over the top, then flip to release the flan along with the caramel sauce. Slice and serve chilled.