Verdict: No, toxic
Onions are highly toxic to cats and should never be fed to them. They contain compounds that can cause severe damage to red blood cells, leading to life-threatening anemia. If your cat has ingested any amount of onion, seek immediate veterinary attention.
N-propyl disulfide causes oxidative damage to red blood cells, leading to the formation of Heinz bodies and subsequent hemolysis (destruction of red blood cells), resulting in anemia.
N-propyl disulfide (a thiosulphate)