My understanding is that onions are bad for dogs.

I have some home made stock that was made with the holy trinity (carrots, onions, celery).  Of course it's all been strained out, but I was wondering if it's still bad to give that stock to dogs or if it's ok.

I think it would depend on volume and your dog and such. My dog does get broths with onions (50lb Dalmatian) but not often. I try and do stews and broths without so i can share them with the pets. but occassionally, i don't worry to much for the dog because it's not a huge amount. i do avoid it with the cats. when i make stews, i take their portions out and then add onions. but my personal cooking is not void of onions, and my dog cleans up after me i got a large 12 qt kettle with the pasta insert for x-mas. i use that for broths now. i put whole chickens in it and make a straight up broth. very easy to lift out the insert with the chicken and have clean broth. you could then sep it, and freeze some clean for the pets and for yourself, add the holy trinity and simmer a bit longer and then freeze.

I make chicken broth all of the time for my dogs but never add onion.   Onion contains thiosulphate which can be toxic to pets.   It doesn't matter if the onion is raw or cooked.  It can produce gastrointestinal problems besides the anemia too & possible death.