I stumbled upon this picture of a common dish around here (although I’ve never seen it with onions):
While I still don’t like the version they serve in the Tongji Cafeteria, I’ve eaten quite good ones in the meantime.
Su-Lin, the cook and photographer, posted the recipe in her blog. I haven’t tried it yet but you can do it. And if you want to enjoy the real Chinese experience just get tomatoes that are not yet ripe or contain black spots or holes and don’t cut them away. :-P