Customer Emma Daniels ordered a baked Camembert at the Severn Shed restaurant in Bristol which was on the menu for £13, but was shocked to discover it was still in the original packaging.
On the supermarket shelves, the French cheese is sold for £1.15.
Ms Daniels wrote on TripAdvisor: “When we go out to eat we realise you pay for more than just the ingredients, you pay for atmosphere, staff, cooking etc etc.
“Even when I cook it at home I cook it in a terracotta dish!!”
Manager Ashley Kirwan told The Caterer magazine: “We have taken action and the chef who was responsible is no longer in the business.
“This is absolutely not normal practice. We don’t do that and there was a genuine mistake. I am not prepared to comment on the ins and outs of it but it happened.
“I have spoken to the guest and explained the situation to her and she is going to come back and dine with us in the future to restore some faith.
“We care about this kind of thing – it is our reputation and my reputation as well and it is going to take a bit of time to build that back up.”