No. You would ask 'Have you finished yet? If you were annoyed you might say, 'haven't you finished yet?' Though it is written as a question the intonation would show that you were annoyed. The phrase 'are you still working on that' is used for things like college work, a project or possibly a prospective romance. 'How's your new novel coming along?' 'I'm still working on it.'

You don't work on your food nor do your food ;) You work on a project and do your homework You could simply say in an informal way beside Brett's suggestions: " Are you still eating?"

Are you not done eating yet?