If someone forgets to say Bismillah before eating, the Sunnah is to say the following Dua as soon as you remember that you forgot to say Bismillah at the beginning, even if partway through the meal:

بِسْمِ اللَّهِ أَوَّلَهُ وَآخِرَهُ

Bismillahi Awwalahu Wa Akhirah

Meaning: "In the name of Allah, at its beginning and at its end."