Although meant to depict "cheese omelette", this phrase is grammatically incorrect. You should say "omelette au fromage", which means "an omelette with cheese".
Despite this, people still say the phrase "for the lols."
Sarah: Dexter's lab?
Mike: Dexter's lab!
Mike: Roflcopter Lolmobile!
Steve Martin does a routine on his 1977 album “Let’s Get Small” where he talks about learning to say “cheese omelet” in French. Armed with this limited knowledge, he enters a restaurant in Paris and proceeds to order a shoe with cheese on it while asking the waiter to force it down his throat.