Yes....every onion that shows up gets eaten up by her.
In this case "toda a cebola" can mean "the entire onion" or "all the onion in the plate".
And "todas as cebolas" mean "every available onion", "all the onionS"
But "toda cebola" is general, any onion she finds, she eats.
This is why I really detest the low frequency, out of context sentences here.
So, if she only eats all of the onion (when she eats onions). would it be, "Ela come toda da cebola?"
I was marked wrong when I wrote, "She eats all of the onion." as the translation, because the other scenarios were sillier. Are we talking about a woman with strange dietary habits? With a mental illness?
She eats all of the onion/all the onion/the entire onion - Ela come toda a cebola She eats all the onions - Ela come todas as cebolas She eats every onion - Ela come todas as cebolas (it's an emphasized statement, just put more emphasis in "todas")
"Ela come cada cebola". It gives emphasis at "cebola", so it's like "You must see what kinds of onions she eats".
"Ela come toda da cebola" i think it's wrong, it's like saying "She eats every from the onion/inside the onion". you must use "tudo" not "toda". "Ela come tudo da cebola".