Are cooked onions dangerous for dogs?

Dogs develop hemolytic anemia if they eat enough onions. I don't think that it matters too much whether the onions are cooked or not. The quantity of onions required is high enough that dogs can generally tolerate small doses of onions without any problem and moderate amounts of onion without clinically apparent disease, even though there may be measurable changes on lab test results. Cats are probably a little more sensitive to onion toxicity than dogs are. I can't find an exact quantity of onions required to cause toxicity problems in dogs, but there are several case reports of onion toxicity and they involve whole onions or sizable portions of chopped onions (like a cup or more). I think that feeding dogs meat that has been cooked with onions is pretty safe but you might want to avoid giving them the broth from around something like pot-roast if there were a lot of onions used in the cooking, just to be safe.

Large amounts of garlic will produce similar toxicity problems in both dogs and cats. I think that the amount required is not likely to be eaten by a cat but there are probably a few dogs who would lap up a container of spilled garlic.

Among common foods, the only other significant toxicity that I can think of are recent reports of toxicity from eating grapes and raisins that have been reported in dogs.