Last month I wrote about how onions are very toxic to cats. Here is a related question....
I know that onions are not good for dogs. But what if you use tomato sauce that was made with onions (it's a thin, consistent sauce - no onions present but was cooked with onions during preparation). Is that safe to use?
I make lots of dog treats for my dog and his buddies and just checking on whether I can use some homemade tomato sauce in a I normally bake.
One of the very interesting things about animal physiology is the differences between superficially similar animals. On the first day of vet school we had one of our most important lessons. And it was pretty simple. "Cats are not small dogs." Our class even had t-shirts printed with this saying. There are some very significant differences in the way dogs' and cats' bodies work, and you can't always assume that what is good or bad for one would be the same for the other. Medications that are perfectly fine for a dog can potentially kill a cat.
Onions are like this. No, they are not good for dogs, but they are nowhere near as toxic as they are to cats. Even onion powder can be potentially deadly to cats, and so should be avoided at all costs. However, only whole onions or whole garlic cloves are potentially dangerous to a dog. Even then it would take quite a bit.
If this was for a cat, I wouldn't take the chance even if it's just small onion particles. However, for a dog, something cooked in onions should be just fine.