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 pan to coat the bottom evenly. Set aside and let the caramel 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.
04 - Pour the custard mixture through a fine-mesh sieve into the caramel-lined baking dish to ensure a silky, lump-free texture.
05 - Place the 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 water bath for gentle, even cooking.
06 - Bake for 60 minutes, or until the center is just set and barely jiggles when gently tapped. Avoid overbaking, as the custard should remain creamy.
07 - 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 firmly set.
08 - Run a thin knife around the edge of the flan to loosen it. Place a large serving plate upside down over the baking dish, then invert in one swift motion to release the flan. The caramel sauce will naturally flow over the top. Slice into wedges and serve chilled.