No rule for this one
No ending or word group predicts the article of Zwiebel. Learn it as a pair and say it out loud: die Zwiebel.
die Zwiebel
It's die Zwiebel. Zwiebel is feminine, plural die Zwiebeln.
Ich brauche eine Zwiebel.
I need an onion.
No ending or word group predicts the article of Zwiebel. Learn it as a pair and say it out loud: die Zwiebel.