Chefs are never wrong -- at least not during working hours. It must have been your hearing. What he said was jardeniere de legumes, which is (more or less) synonymous with assortis de legumes. He might also have said, macedoine. If chef said it, you're mistaken and not him.
Chef or not, in the future if someone who is not French asks you if you speak French, the correct answers is "J'regrette beaucoup, j'suis tres desole, I assure you. Mais un petit peu de Franglaise only." Make a little pinching gesture when you say the word petit. Then ask very innocently, "Do you?" Just get the words out with worrying too much about accent. This goes double if you or the asker has any sort of English accent, as Englishmen, no matter how fluent in another language, are exepected to impose their own voicing. It's a holdover from the days of empire.
Your other co-worker is pretending to be mixed up by manger, which is part of garde manger. Rest assured he is a gentle jokester and not total idiot pretending to know what he doesn't.
As a noun, "manger" means food. AFAIK (and I'm not really fluent in French), there's no other word which could possibly be pronounced marn-jair, not even by brummy.