The incident happened on Wednesday, 2 June at about 7.30pm
The victim, a man in his early 30s, was with a friend outside a fish and chip restaurant in Stoke Newington High Street, N16.
Two men were entering the restaurant. One was wearing a clown mask.
The victim made a joke that they would not let him in wearing that mask and the man took offence.
The two men then followed the victim and his friend from outside the shop and one of them threw a metal chair at him.
The victim was then set upon and repeatedly punched in the face.
Both suspects made off and police and London Ambulance Service were called, the victim was taken to hospital.
As a result of the assault the victim suffered a fracture to his cheekbone, a broken nose, a broken finger and has lost the sight in one eye.
The suspects are described as two black males.
There have been no arrests.
Detective Constable Gary Holland of the Central East Command Unit, said: “A simple joke had led to a man being so badly beaten that he has lost sight in one of his eyes. How can it be that a trivial remark has led to such terrible consequences?
“Any rightminded person reading this will know how wrong this is and I am asking anyone who knows anything about this assault to call police. You can also call Crimestoppers if you do not want to give your name, but please do call and tell us what you know.”
Any witnesses or anyone with any information is asked to call police on 101 or contact via Twitter @MetCC. Please quote CAD 7077/02JUN