Preheat oven to 350ºF (180ºC).
In deep heavy saucepan, stir sugar with 2 tablespoons (25 mL) water over medium heat until dissolved, brushing down side of pan with pastry brush dipped in cold water. Bring to boil over medium-high heat; boil vigorously for about 8 minutes or until dark amber. Immediately pour into flan mould or 4 cup (1 L) oval baking dish.
In separate heavy saucepan, heat milk until tiny bubbles form around edge. Meanwhile, using small knife, split vanilla bean in half lengthwise; scrape out seeds. Add seeds and pod to milk along with sugar, and salt; simmer for 15 minutes or until reduced to 3 1/2 cups
Combine eggs, egg yolks and orange and lime rinds. Pour milk mixture through
cheesecloth-lined sieve into egg mixture; whisk to combine. Pour into the caramel-coated mould.
Place mould in larger roasting pan; pour in enough hot water to come halfway up side. Bake in centre of oven for about 1 hour or until firm and cake tester inserted in centre comes out clean. Transfer roasting pan to rack; let cool on rack until at room temperature. Remove from water to let cool completely. (Make-ahead: Cover and refrigerate for up to 2 days. Place mould in warm water for 5 minutes before turning out.)
Run knife around edge of mould; invert onto serving platter.