What should a person say when he realizes after he has finished eating that he forgot to say Bismillaah?

What is prescribed for the one who forgets to say Bismillaah when he first starts to eat is to say when he remembers, Bismillaah awwalahu wa aakhirahu (In the name of Allaah at the beginning and the end) or to say Bismillaah fi awwalihi wa aakhirihi (In the name of Allaah at the beginning and the end). 

It was narrated from ‘Aa’ishah (may Allaah be pleased with her) that the Messenger of Allaah (peace and blessings of Allaah be upon him) said:  

“When one of you eats, let him remember Allaah at the beginning, and if he forgets to remember Allaah at the beginning, let him say Bismillaah fi awwalihi wa aakhirihi (In the name of Allaah at the beginning and the end).” Narrated by al-Tirmidhi (1857), and he said: it is hasan saheeh. 

The phrase “and if he forgets to remember Allaah at the beginning, let him say…” includes both the one who remembers to say Bismillaah whilst eating, or at the end, or shortly after he has finished eating, because of the general meaning of the hadeeth. 

It says in Kashshaaf al-Qinaa’ (5/173): The apparent meaning is even if that is after he has finished eating.

 It says in Mughni al-Muhtaaj (4/411): 

If he forgets to do it (i.e., say Bismillaah) at the beginning, he may say it partway through, and if he forgets it partway through, he may say it at the end. End quote. 

It says in Nihaayat al-Muhtaaj (1/184): 

He should not say it (i.e., Bismillaah) after finishing wudoo’, which is unlike the case with eating, where he may say it afterwards. End quote. 

It says in al-Haashiyah ‘ala Nihaayat al-Muhtaaj: 

Meaning, that if the time between finishing his meal and remembering is not too long, he may say it. What is too long is to be determined by what is customary. End quote. 

The hadeeth also indicates that saying Bismillaah fi awwalihi wa aakhirihi (In the name of Allaah at the beginning and the end) takes the place of saying Bismillaah at the beginning, and the reward for seeking the help of Allaah when starting to eat will be recorded for the Muslim, as a honour and a bounty from Allaah, may He be blessed and exalted. 

Al-Munaawi said in Fayd al-Qadeer (1/296): 

How can it be true that one has sought the help of Allaah at the beginning when he did not do that at the beginning?  

Because we say that the phrase indicates that this is the case, thus the one who utters this phrase becomes like one who sought help at the beginning and will get the same blessing as one who sought help at the beginning. End quote. 

The Sunnah for the one who has finished eating is to praise Allaah for it in different ways, such as the following. 

It was narrated from Abu Umaamah (may Allaah be pleased with him) that when his meal was cleared away, the Prophet (peace and blessings of Allaah be upon him) would say: “Al-hamdu Lillaahi hamdan katheeran tayyiban mubaarakan fihi ghayri makfiyyin wa laa muwadda’ wa laa mustaghni ‘anhu Rabbanaa (Allaah be praised with an abundant, beautiful, blessed praise. He is the One Who is Sufficient, Who feeds and is never fed, The One Who is longed for, along with that which is with Him, and the One Who is needed. He is Our Lord. Narrated by al-Bukhaari (5458). 

So if a person forgets to say Bismillaah when he starts to eat, then he remembers it when he has finished eating, it is mustahabb for him to say Bismillaah fi awwalihi wa aakhirihi (In the name of Allaah at the beginning and the end) and then praise Allaah. Thus he will have acted in accordance with both hadeeths, and obeyed the command of the Prophet (peace and blessings of Allaah be upon him). There is no contradiction in saying both dhikrs after one has finished eating, and it seems that there is nothing wrong with saying al-hamdu-lillaah before saying Bismillaah fi awwalihi wa aakhirihi (In the name of Allaah at the beginning and the end), in sha Allaah, because the aim is to say both dhikrs, the Tasmiyah (saying Bismillaah) and praising Allaah, so it does not matter which of them one starts with. 

And Allaah knows best.