"Einen Teller Suppe?"
I assume because it's shortened from something like "may I have a plate of soup?". Or perhaps used in response to a question such as "what do you want to order?". In both cases, "a plate of soup?" is in the accusative case.
Of course, the lack of context here doesn't help!
I don't know about you but In 56 yrs. I've never had a plate of soup!! Wow, I think I'll give that a try today. There must be a special spoon for that! Perhaps, a Loeffeltel. lol