"When you are about to eat, you should say:

بِسْمِ اللَّهِ

In the Name of Allah.

Bismillaah.

...and if you forget to say it before starting, then you should say [when you remember] :

بِسْمِ اللَّهِ فِي أَوَّلِهِ وَآخِرِهِ

In the Name of Allah, in the beginning and in the end.

Bismillaahifee 'awwalihi wa 'aakhirihi.

Abu Dawud 3/347, At-Tirmithi 4/288. See Al- Albani's Sahih At-Tirmithi 2/167.