Symbolism, Imagery, Allegory
"The Onion Cellar" is a nightclub where Oskar lands a part-time gig as a drummer. People pay good money to come to the club, cut up onions, and have a good, cathartic sob fest. The fact that this happens with a club full of other people turns the whole thing into a sort of group therapy, but noisier.
People wept. At long last, people wept again. Wept openly, wept without restraint, wept honestly. The dew fell […] Dams bursting in spring tide. (42.25)
The key words in the above quote are "at long last." Obviously something or things have happened that the club-goers haven't been able to remember and cry about. The onions provide a trigger for everyone in the room to let out their collective sadness. Everyone's doing it, so no one feels judged. The people in the club recall their childhoods and cry about lost loves and regrets. But we suspect that Grass thinks that German society needs to have a few visits to The Onion Cellar to bring up and cry about the memories of Hitler's Germany. It's an absurd but brilliant symbol, and Grass provides the onions, no charge.