Siroosh Bavarsad, 31, was annoyed when he heard Graeme Noble complaining about his Glasgow establishment The Clock Tower on the day it opened. Bavarsad snapped after weeks of mounting pressure leading up to the opening day.
He headbutted Mr Noble, 57, as he complained, knocking him unconscious before walking out.
Onlooking medics who were on a lunch break from the nearby Royal Infirmary hospital tended to Mr Noble, a train driver.
At Glasgow Sheriff Court, Bavarsad, from Hyndland, Glasgow pled guilty to assaulting Mr Noble to severe injury and permanent disfigurement on March 6 last year.
Procurator-fiscal depute Pauline Henderson said: "The complainer approached the desk of the restaurant to complain about the standards of service.
"The accused is described to have flown into a rage and headbutted the complainer once to the face, which knocked him to the floor. A number of medical staff were having lunch in the restaurant that day and immediately tended to the complainer and summoned both ambulance and police.
"The complainer was put in the recovery position and was initially conscious, but lost consciousness for a number of minutes."
The court was told Mr Noble was left with a scar above his eye.
Father-of-two Bavarsad owns a number of businesses in Scotland. Defence lawyer Stephen Fox said: "March 6 was his first day of opening and despite weeks of training I think it's fair to say everything that could go wrong was going wrong on that particular day.
"It was extremely busy for the opening, essentially the staff couldn't cope. The complainer, as I understand it, had approached Mr Bavarsad prior to this and made a number of complaints including the amount of napkins on the table and the fact he hadn't received a roll with his soup."
He said his client told the customer it was the opening day and they would try to do better, before going into the kitchen.
He added: "When he was there he heard raised voices, he then re-attended the restaurant, where the complainer was now complaining to Mr Bavarsad's mother. His mother is not a member of staff - she was only there to assist with the opening day."
"There was an exchange of words and Mr Bavarsad advises me that all of the weeks of pressure leading up to March 6 and pressure of the day and seeing his mother being upset at the complaint being made, he lost his temper and frankly lashed out."
The lawyer said Bavarsad was found nearby, sweating and hyperventilating because he could not believe what he had done and he planned to return to apologise - unaware of Mr Noble being knocked unconscious.
Sheriff Martin Jones, QC, deferred sentence until next month for reports and continued bail.