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The question is not about how much beef he ate, but how much onion.
I found one source that lists the critical dose for a single ingestion as 0.5% of body weight, which for your dog is 25g. If you used the onion in the usual ratios for a dish/sauce and your dog ate just the meat, it should pretty harmless. If on the other hand you made a dish that features onions as a main component (a typical example would be goulash, which uses roughly equal parts of onions and meat), it may be a different scenario.
Then consider that while onions (or rather all alliums) can be poisonous, it’s not a black-or-white issue.
There are two main effects of alliums:
Even severe indigestion (watch out for excessive drooling, signs of intestinal pain, vomiting or diarrhea) is usually not fatal but may require treatment.
The second effect is similar to blood loss due to an external wound – ranging from unnoticed over tiredness to collapse.
In short, do the maths of how much onion your dog really ate (beef doesn’t “soak up” much, the flavor is concentrated in the sauce), contact your vet if the dog really had much. Otherwise keep an eye on your pet and I would probably take it a bit slower over the next days just in case something is amiss.
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How much cooked onion can hurt a dog?
Not surprisingly, pets actually have to eat the onions to get sick, but depending on their size, they may not have to eat much. One fourth of a cup can make a 20-pound dog sick while several cups may be needed to make a large dog sick.
Can Dogs Eat Green Onions or Chives?
Green onions (Allium fistulosum) and chives (Allium schoenoprasum) are both members of the Allium genus and are also toxic to dogs. Dogs should not eat any form of onion—red, white, yellow, sweet, green onions, chives, leeks, or even garlic—as these are all members of the Allium genus.