Camera aspect (and other camera calls) return wrong values on Start / Awake / OnEnable

It seems in Unity 6 a lot of the Camera API returns garbage values in Start / Awake / OnEnable (at least on mobile devices, but it does it in the editor too).

More specifically, I’m struggling with Camera.aspect, which returns a wrong aspect ratio until a frame or two has passed, which is too late for what I’m doing.

What is the way to call Camera.aspect reliably with Unity 6?