Jon was appearing on the show with his wife Lucy Beaumont.
They'd gathered around the cooking station to test ghee-friend prawns and a Masoor dhal.
Jon stood back while the others tried the prawns, before host Simon Rimmer assured him that the dhal was vegan.
But in reality, other ingredients had been mixed into the chickpea-based dish, including ghee - a type of butter made from cow's milk.
After an ad-break, Simon was forced to apologise to Jon after realising their mistake.
He told viewers: 'Earlier on in the previous cooking item I may have informed guests that the dhal was in fact vegan.
'There was, of course, ghee used in the recipe, which isn’t vegan.
'I allowed Jon Richardson, who is indeed a vegan, to eat some of it.
'On behalf of myself and the Sunday Brunch team I would like to sincerely apologise and hope I haven’t offended you and spoiled your life.'
But while Jon was good-natured about the incident, some viewers weren't satisfied by the apology.
One Twitter user wrote: 'I'm really offended by the ‘joking’ apology and the laughing in the background. It would literally make me sick. You own a vegan/vegetarian restaurant!'
Another said: 'Typical! #SundayBrunch 'apologised' for telling us ghee is vegan, shows we can't trust how seriously you take us, courts have ruled in our favour so it's about time you took notice!!'