My childhood cats (and dog) were always given whatever was left of the Sunday dinners EVERY sunday for 15 years, including onions. It was the norm then to give pets left overs.
As well as 15 years of onions, these cats went outdoors, never had vaccinations (were neutered!) and rarely had a vets visit, and lived to old age respectively! Just demonstrating that although we know what is best for our kitties now, it wasn't always so and cats have coped for centuries!