The fast food worker, a 41-year-old man, was attacked behind the counter of the Burger King restaurant at Marylebone Station on a Thursday evening.
The victim was assaulted by a man who became angry when he was told he would have to pay for a drink if he wanted a paper cup, police said.
Officers said the customer, who had pushed into the queue, refused to pay and walked behind the counter where he punched the staff member in the head twice.
He then threw the victim onto the floor and tried to stamp on him before leaving the scene, according to police.
The victim was left with grazes to his legs and bruising around his eye.
British Transport Police officers are appealing for witnesses to the attack, which took place at about 9.20pm on March 9.
Officers have released a CCTV image of a man they want to speak to in connection with the assault.
Investigating officer, Det Con Martin Coburn, said: “I would like to speak to the man shown in the CCTV image as he may have information which could help with the investigation.
“This was a violent attack against a member of staff who was simply doing his job and it is vital that we find the person responsible.
"Fortunately, the victim has recovered from his physically injuries, which in the end were not serious, but they could have been a lot worse as the offender was trying to stamp on the victim whilst he was on the floor.”
Anyone with information about what happened or who recognises the man shown in the CCTV image, is asked to call 0800 40 50 40 or text 61016, quoting reference number 211 13/04/2017.