Onions, whether fresh, cooked or dehydrated, cause anemia in monkeys.
I thought it would be a good idea to educate owners about onions.
Onions are known to cause fatal hemolytic anemia.
Heinz-body hemolytic anemia. What is anemia? Anemia is any condition in
which the number of red blood cells is decreased. Red blood cells carry oxygen
to the tissues on a molecule called hemoglobin.
Hemolysis is the destruction of RBCs. Heinz-body changes to RBCs is irreversible.
What clinical signs will occur with onion toxicosis? The signs that you see will
depend on the amount of onions consumed, the length of time they have been fed, and the size of the animal. Gums (mucous membranes) may appear pale if
there is moderate to severe anemia/ if lots of RBC have ruptured, there may be
hemoglobin in the urine, causing it to appear reddish or brown, or the tissues
may become jaundiced. Weakness, depression, rapid heart rate and rapid respiratory rate are observed as a result of there not being enough oxygen reaching the tissues (hypoxia). Vomiting, decreased appetite and diarrhea may also occur.
Some monkey owners feed All-meat baby foods, or stage one chicken sticks. However, let's go down the list of ingredients: chicken, water, calcium, reduced dried skim milk, salt, sugar, onion powder, and garlic powder.
Onions are often added to products to enhance the flavor of many foods, especially for human baby's, so be sure to read the label carefully before feeding any prepared food to your monkeys.
What about onion toxicity in humans? Since damage from feeding onions seems worse in smaller animals, it may be that the amount of onion in baby foods is not enough to cause problems in humans, in relation to their size. But certainly, the amount of onion in baby foods can be potentially harmful to little marmoset, tamarins and other monkeys. And fresh or cooked onions will certainly cause a degree of Heinz-body formation in the blood of primates. 
So, please spread the word to other owners that onions should not be fed to non-human primates or other species of animals, as well.