A court heard that the chain had been selling Aragosta e Gamberoni, a lobster and king prawns pasta dish, since 2014.
But the BBC reports an investigation found that the menu item was not made from chunks of lobster but a cheaper mix that was 35% lobster, 34% white fish, and other ingredients.
Swansea Trading Standards alerted the chain to the issue in March 2019, and the restaurant group removed the item from its menu.
ASK was fined £40,000 on Tuesday (14 January) for misleadingly describing its food.
The restaurant group admitted the charge at Swansea Magistrates' Court in November.
It apologised for the ‘error’ but denied it had a financial motivation behind the labelling.
ASK said the menu item, which sold for £14.95 but had raw ingredient costs of £2.84, had the lowest profit margin of all its pasta dishes.
The casual dining chain is owned by Azzuri Group, which also operates the Zizzi and Coco di Mama restaurant brands.