After a good search of the internet I could not actually find the ingredients of the broth. I think as its a broth your dog will probably be perfectly fine, I think the onion's must not have an affect when boiled and strained. Source ->

I would however be more worried about the sodium content of these 'enhanced' chicken breasts. I've read on one page that the sodium in some chicken is 20% (440 mg) of an adult human's daily intake which I'm no vet but I'm guessing is close if not way more than a dog should be having. Source ->

Might be worth just buying premium dog food?