The inscription above is the seal of the Sultan of Ottoman Empire, or Tughra. Each Sultan has its own Tughra, but generally follow the same pattern as in the picture. The symbol is actually the Sultan’s name, written in a beautiful calligraphy. Each line depicts one or more words, as seen in below animation.
You will find a lot of different Tughra in Topkapi Palace, the palace of Ottoman Empire. Nowadays, I think the symbol is used as the mark of Ottoman Empire and Turkey in general, just like the restaurant where we ate.
Written in response to WordPress Photo Challenge: Names