01 - Preheat oven to 350°F. Position the rack in the center of the oven.
02 - In a heavy saucepan, combine 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 to evenly coat the bottom and sides. Set aside 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 smooth and homogeneous.
04 - Pour the custard mixture through a fine-mesh sieve into the caramel-lined baking dish. This ensures a silky, lump-free texture in the finished flan.
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 gentle water bath for even 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 - Remove the flan from the water bath and allow it to cool to room temperature. Once cooled, cover and refrigerate for at least 3 hours or until thoroughly chilled.
08 - Run a thin knife around the edge of the chilled 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 will flow over the top as a glossy sauce. Slice into wedges and serve chilled.