01 - Preheat the oven to 350°F.
02 - In a heavy saucepan, combine granulated sugar and water. Cook over medium heat without stirring until the sugar dissolves and turns a golden amber color, about 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 harden.
03 - In a large bowl, whisk eggs, egg yolks, and salt until combined. Add sweetened condensed milk, evaporated milk, whole milk, vanilla extract, and orange zest if using. Whisk until smooth.
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 halfway up the sides of the flan dish to create a water bath.
06 - Bake for 60 minutes, or until the center barely jiggles when gently tapped.
07 - Remove the flan from the water bath. Cool at room temperature, then refrigerate for at least 3 hours or until thoroughly chilled.
08 - Run a thin knife around the edge of the dish. Invert a large serving plate over the top, then flip to release the flan and caramel. Slice and serve chilled.