The Tzemach Tzedek once told someone to become an onion.

This was a person who had been traveling from town to town spreading chassidic teachings. But now he had stopped. All the fuss over himself and his talents, he claimed, had been feeding his ego. Now he had to work with himself to diminish that negative aspect of his character.

The Tzemach Tzedek showed no pity. “An onion should become of you,” he cried, “but teach others what you must teach them!”

Why an onion?

An onion is thrown into the pot not in order to eat the onion, but only to give flavor to the chicken and the broth. Like the onion, you sometimes need to sacrifice your own personal growth so that others may grow.

Toras Menachem 5717, Vol. 1 p. 313
Translated in Bringing Heaven Down to Earth
by Tzvi Freeman