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)