Our most powerful flan. Yolks only for a dense, almost set-cream texture, a properly dark burnt caramel, and a long slow water bath. Faintly intimidating, reserved for occasions that deserve a custard with opinions.
Take the sugar and water past golden to a deep mahogany amber, right to the edge before bitter. This depth is the whole point of Opus. Pour into a 22cm dish and coat the base.
Warm the milk and cream with the vanilla pod, seeds, and orange zest. Hold just below a simmer for a few minutes, then leave to infuse 20 minutes. Strain out the solids.
Whisk the yolks and sugar until pale. Pour the warm dairy in very slowly while whisking so the yolks do not scramble. Strain twice through a fine sieve.
Pour over the caramel. Set in a deep tray, surround with hot water to two-thirds height, and bake at 150C / 300F / gas 2 for 70 minutes. The centre should still tremble.
Cool fully, then chill overnight. Turn out only when fully cold. Expect a glassy, dark caramel layer and a custard that holds its shape but yields instantly.