I have this metal knob, sitting on a metal plate. The knob is lit well, but the plate underneath is not.
And the plate should be since its a bright metal material.
There are no lights in my scene, just a HDRI. Any ideas ?
The flatness of the object is the cause. Turn down your hdri and add a light over the knob and panel that adds specular highlight to the panel but not illuminate the knob so much. Then turn up the hdri to get it back to your desired look.
This isn’t going to work as you want with only one light source.
Someone may come along to show better.