I don't recommend feeding onions to your chickens because any potential health benefits are far outweighed by the potential health risk.

Can Chickens Eat Onions?

No chickens shouldn't eat onions. Onions, and other members of the allium family, contain a toxin called thiosulphate.

Thiosulphate destroys red blood cells faster than they can be rebuilt. It damages the cells and prevents them from carrying oxygen to other parts of the body. 

This can cause anaemia and jaundice in chickens (and also in dogs).

Symptoms of toxicity can include rapid breathing, lethargy, diarrhea, and overall weakness. Excessive amounts can even lead to death. 

Other alliums include shallots, chives, leeks and scallions.

What About Garlic?

Garlic, which is in the same allium family as onions, contains only a fraction of the thiosulphate as onion does and has some truly amazing health benefits, including supporting immune and respiratory systems. 

And once dried and processed, powdered garlic has only negligible amounts of thiosulphate in it, so I feel very comfortable adding garlic powder to our chickens' daily feed or adding fresh garlic cloves to our chickens' water in the amount recommended by the experts. 

I do give my chickens garlic powder in their daily feed. They would have to eat an awful lot of fresh garlic for it to cause them any negative health issues.

Reference: https://chickendvm.com/poisonous/onion

Here is a comprehensive list of toxic foods that should be avoided. 

And a list of healthy treats for chickens.  

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