I have a sunken cheek on one side of my cheeks. What treatment would you recommend? (photo)

An examination of your face or more complete photos would be helpful in giving you the best advice.  I am also not sure of the cause of the sunken area of one of your cheeks.  If there are no medical issues involved, it is possible that a filler could be used to add volume to the sunken area and improve it.  

Autologous Fat Transfer.- that is using your own fat taken from another part of body(usually abdomen) to fill the sunken area.

Thank you for posting a
photo. If you have a loss of volume I recommend learning more about Voluma.
Voluma is a facial filler that will instantly volumize the cheek area. This new
injectable is said to last two years. Consult with your doctor to make sure
this is the best option for you.

It is hard to give you advice from these photos, as they are hard to evaluate.  I can see the small indentation on one cheek.  This could be filled with something as simple as a filler which I do everyday.  Without better photos, however, I can't really give you much more information.