Being very new to the semi-pro A1, my old Sony HC3 handy cam had a very useful 'Back light' button for shooting people/object silhouetted by a bright back light.

What is the best one-push way to achieve this using the A1?

The absolute best way is not a one-push solution (sorry). Keep the iris on manual (using the exposure lock button), when you are in tricky situations. In 'even lighting' auto might be fine, but you don't want the camera making all the adjustments for you. That is one of the things that separates the pro work from amateur. Get used to using zebras, and know how to expose for faces. In the end...you'll benefit from much better footage.

Oren,

Thanks for that. I thought going manual was the only option

The only thing a "backlight" function does is to shift the exposure. Manual is the best way to do this but if you want the camera to do it for you you can use the AE shift control in the camera setup menu. There you can select from 15 different levels. for back light you would want to set a + level. you can start with +1 and see what works best in your given situation.

Having said that, learning to use manual exposure and zebras is the best way.