01 - Preheat the oven to 350°F. Position the rack in the center of the oven.
02 - In a heavy saucepan, combine the granulated sugar and water. Cook over medium heat without stirring until the sugar dissolves and turns a deep golden amber color, approximately 7 to 9 minutes. Immediately pour the hot caramel into a 9-inch round baking dish or flan mold, tilting the dish to evenly coat the entire bottom. Set aside and allow the caramel to 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 completely smooth and homogeneous.
04 - Pour the custard mixture through a fine-mesh sieve into the caramel-lined baking dish to remove any air bubbles or solids, ensuring a silky, smooth final texture.
05 - Place the filled flan dish inside a larger roasting pan. Carefully pour hot water into the roasting pan until it reaches halfway up the sides of the flan dish, creating a gentle water bath for even, gentle cooking.
06 - Bake in the preheated oven for 60 minutes, or until the center is set and barely jiggles when gently tapped. The edges should be firm while the center retains a slight wobble.
07 - Carefully remove the flan dish from the water bath and allow it to cool to room temperature. Then refrigerate for at least 3 hours, or until thoroughly chilled and fully set.
08 - Run a thin knife around the inner edge of the dish to loosen the flan. Place a large serving plate upside down over the dish, then quickly invert. The flan should release onto the plate with the caramel flowing over the top. Slice into wedges and serve chilled.