Write “Dear Commissioner” and their last name as a salutation. Don't stress about sounding too formal or casual—with letter headers, “Dear” works perfectly fine as a salutation. Use “Commissioner” and their last name, and skip over their first name altogether.
Most formal letters will start with 'Dear' before the name of the person that you are writing to. You can choose to use first name and surname, or title. and surname. However, if you don't know the name of the person you are writing to, you must use 'Dear Sir or Madam,'.
Their full name, followed by their title; for example, “Mary Black, Assistant County Attorney,” with a salutation of “Dear Ms. Black.”