I would like to know the first food item that will be offered in heaven.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

The first meal that is presented to the inhabitants of the paradise is fish-liver (fish in the narration refers to either regular fish of whales) as reported in the narration of Anas "When the news of the arrival of the Prophet, Sallallaahu alayhi wa sallam, at Madeenah reached 'Abdullaah Ibn Salaam, he went to him to ask him about certain things, He said, "I am going to ask you about three things which only a Prophet can answer: What is the first sign of The Hour? What is the first food which the people of Paradise will eat? What makes a child resemble his father or his mother?" The Prophet replied, "Jibreel has informed me of that." Ibn Salaam said, "He (i.e. Jibreel) is the enemy of the Jews amongst the angels". The Prophet said: "As for the first sign of The Hour, it will be a fire that will collect the people from the East and force them towards the West. As for the first meal which the people of Paradise will eat, it will be the caudate (extra) lobe of the fish-liver. As for the child, if the man's discharge precedes the woman's discharge, the child's looks will resemble the man, and if the woman's discharge precedes the man's, then the child will resemble the woman."

On this, 'Abdullaah Ibn Salaam said, "I testify that none has the right to be worshipped except Allaah, and that you are the Apostle of Allaah." and added, "O Allaah's Apostle! Jews invent such lies as make one astonished, so please ask them about me before they know about my becoming a Muslim".  The Jews came, and the Prophet said: "What kind of man is 'Abdullaah Ibn Salaam among you?" They replied, "The best of us and the son of the best of us and the most superior among us, and the son of the most superior among us." The Prophet said: "What would you think if 'Abdullaah Ibn Salaam should embrace Islam?" They said, "May Allaah protect him from that." The Prophet repeated his question and they gave the same answer. Then 'Abdullaah came out to them and said, "I testify that none has the right to be worshipped except Allaah and that Muhammad is the Apostle of Allaah!" On this, the Jews said, "He is the most wicked among us and the son of the most wicked among us." So they degraded him. On this, he (i.e. 'Abdullaah Ibn Salaam) said, "It is this that I was afraid of, O Allaah's Apostle." [Al-Bukhari]

Allaah knows best.