Hi I'm Rosie one of the vets and I'd like to try and help you and your dog. You are correct - if he ate it over five hours ago then it would be absolutely pointless trying to make him sick at this point. However, there is still a chance that he only just ate it before you came home in which case there would be value in taking him to a vets. I used to run an emergency clinic, and we had a policy of when the time the dog (or cat, for that matter) ate something poisonous we used to make them vomit anyway, as there is always the chance they ate it recently.
Having said that, you are lucky because he is such a big dog. For a 120 pound dog he would need to eat approximately one and a half pounds of onions for it to be a toxic
dose. If he had eaten a bag of onions then I think you may be in trouble. I doubt that a loaf of onion bread would contain that much onion, to be honest. If you have another one then it would be worthwhile checking.
Given the above I think that keeping an eye on him at home over the next few days would be fine. If he had ingested a toxic dose then you usually see them drooling a bit, possibly with some vomiting (which may not be a bad thing, given the circumstances) and diarrhoea. If it starts to give him anaemia this can take between one and five days to fully appear. If his gums go pale, he goes off his food, increased heart rate, increased lethargy or goes a bit wobbly then I would get him to your vets. In the long run, even if he does get a bit anaemic (which I doubt) then they do really well with supportive care - i.e. going on a drip and possibly a blood transfusion, provided you get him to the vets if you get worried.
I hope I've helped - if you have any questions from this then just hit 'reply' and I will get them and get back to you. Otherwise, if you have found this useful, please leave a positive rating so that I may be compensated for my time. Thank you, ***** ***** luck, Rosie.