Put oven rack in middle position and preheat oven to 350° F.
In a small, heavy saucepan, cook the sugar over medium heat, without stirring, until it begins to melt. Continue to cook, stirring occasionally, until the sugar turns a deep golden color. Immediately pour into a 9-inch round ceramic or glass baking dish or a metal cake pan that's at least 2 inches deep. Tilt the dish to coat bottom (use caution, the dish will be hot). Cool until hardened, 10 to 15 minutes.
Blend the remaining ingredients in a blender, until smooth. Pour custard through a fine-mesh sieve over caramel in dish, then transfer dish to a large roasting pan and place it in the oven. Pour enough boiling-hot water into roasting pan to reach halfway up the sides of custard dish. Bake until custard is set but still wobbly in center when gently shaken and a knife inserted in center comes out clean, 1 to 1 1/4 hours. Transfer dish to a rack to cool completely, about 40 minutes. Chill flan, covered, until cold, at least 8 hours.
To unmold flan, run a thin knife around edge of dish to loosen the custard. Invert a large platter with a lip over the baking dish. Holding dish and platter securely together, quickly invert and turn out flan onto platter. Caramel will pour out over and around flan.