If one forgets to recite the du‘aa before eating, then one should recite the following du‘aa when one remembers:
بِسْمِ اللهِ أَوَّلَهُ وَ آخِرَهُ
(I commence) in the name of Allah Ta‘ala from the beginning till the end.
( Tirmizi #1858, Haakim #7089 )