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:
- Irritation of the GI tract and
- Destruction of red blood cells.
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.