Is цибуля singular or plural? It said "цибуля" = onion, and now it says "onions".
Цибуля is a collective noun, it has the form of a singular noun but refers to both singular and plural. Морква (carrot/carrots) and some other food words work the same way.
Ok, then would it be an error to treat it as a singular noun in this sentence's translation? Ex: Yes, an onion. Do you eat an onion?
I don't think it'd be an error. But I think just omitting the article or using the plural gives it that same sort of collective noun feel to English: "Yes, onion. Do you eat onion?"
Say, you spotted an onion in your food and you ask the server: is that onion? She would say: yes, it's an onion. Are you allergic to onions?
This is accepted and is correct. Skipping "an" in English would be wrong. Wouldn't it? (i.e. like in the example below by @jmango2)
Skipping "an" would not be wrong. The use of "an" would be appropriate if someone were to eat the onion in the same manner as an apple but not when onion is one of the ingredients of a dish because it is pieces of onion and not a whole.
I think this would be the „vocative case”.
This does not make sense in English: Yes onion.
I think it could also be: Yes onions. Do you eat onion?
For the first sentence так could also be translated as OK