An onion is NOT a TYPE of vegetable. It is a specific living thing, not a species. You can say any of the following: "The onion is a vegetable" has two possible interpretations: "onion" could name the species, or "the onion" could be one particular onion that you are, say, holding in your hand. "Onion is a type of vegetable" (Here "onion" names the species.) "Onion is a vegetable" (same as previous) "Onions are vegetables" "An onion is a vegetable" (This is true because every single onion is, in fact, a vegetable. Instead of "an onion is", you could say "every onion is" or "all onions are".) " 'Onion' is the name of a vegetable"

I usually would make it plural such as onions are a vegetable

Either "an onion" or "the onion" could work. "The onion" is more formal. You could also make it plural: Onions are a type of vegetable. But since "onion" is countable, what you can't do is use it in the singular without an article or some other determiner.

The onion is a type of vegetable.