No — dangerous
No. Onions — raw, cooked, powdered or in leftovers — damage dogs’ red blood cells and can cause life-threatening anaemia. All forms are dangerous.
Onions (and the whole allium family: garlic, leeks, chives, shallots) contain N-propyl disulfide, which oxidises haemoglobin in a dog’s red blood cells, making the cells rupture. The damage is cumulative and can come from one large meal or repeated small doses — onion powder in baby food, gravy, soup, pizza or takeaway leftovers is a classic hidden source. Toxicity starts around 0.5% of body weight in onions: just 100 g of onion (one medium onion) can sicken a 20 kg dog.
None — and watch the hidden sources. If your dog ate onion or onion-heavy food, call your vet: signs of anaemia may not appear for several days, and your vet may want blood tests rather than waiting for symptoms.
If you want to flavour your dog’s meal, a spoon of plain bone broth (onion-free!) or plain cooked meat does the job safely.
This article is general information, not veterinary advice. If your pet has eaten something potentially harmful or shows symptoms, contact your vet or an emergency clinic immediately. Full disclaimer.