"This is an onion."
Translation:Das ist eine Zwiebel.
"dies" would be possible, but very formal.
If you want to emphasise that "this is an onion, not that" (or just to add random emphasis), you can say, "Das hier ist eine Zwiebel."
"Diese ist eine Zwiebel" doesn't work for this purpose. (It would work for "Cinderella fährt in einer Kutsche. Diese (= die Kutsche) ist eine Zwiebel." "Cinderella rides in a carriage. The carriage (literally: 'this one') is an onion.")
"dies" = "this [here/there]" is like pointing a finger at something and address the whole [collection of] thing[s] as "this".
"Dies ist eine Zwiebel." = The thing I'm holding is called an onion.
"He only scored 45%. Dies bedeutet, dass er die Prüfung nicht bestanden hat." = This fact means that he didn't pass the exam.
"She said she had paid for the books. Dies lässt sich nicht mehr nachprüfen." = What she claims to have happened cannot be checked/verified anymore.
Note that in the last two examples, "dies" doesn't refer to an object but to the whole statement that came before.
"diese(r/s), pl. diese" specifically refers back to an object (or several, if plural) mentioned before, as in the Cinderella example, and takes the same number and gender:
"Cinderella fährt in einer Kutsche (f.). Diese (f.) sieht aus wie eine Zwiebel." = The carriage I mentioned earlier looks like an onion.
"Der Wecker benötigt zwei AAA-Batterien (pl.). Diese (pl.) müssen separat erworben werden." = "The alarm clock needs two AAA batteries. These need to be bought separately."
It might also help to look at how you ask after "dies" and "diese(r/s)":
Was ist schön? Dies (or: das [hier]) ist schön. - What is lovely? This is.
Welche(r/s) ist/sind schön? Diese(r/s) ist/sind schön. - Which one(s) is/are lovely? This one is. / These are.
Duo never gives context and usually accepts both answers as correct when this happens. I presumed like in numerous other lessons that either would be acceptable; in addition, my understanding is that Dies is usually used if the object noun is plural, as Dieser, Diese, and Dieses are reserved for singular items. I think considering Duo's frequent rule of accepting both when context is not given, Diese ist eine Ziebel should be accepted. The only possible issue for new learners would be the confusion with the -e ending, before one learns that the endings only apply to singular nouns, would be that a newbie would think Diese was plural. However, in this case, you're stating that this is only acceptable when Diese is referring back to an earlier reference to the "onion" in the previous clause, or sentence. Which is the first I've heard it, and contradicts all the previous acceptance by Duo for both cases.
I'll admit, it's great to know this 'rule' and perhaps NOW it is being introduced to me. But I hate to have to unlearn all my lessons on using dieser, diese and dieses and using Dies for plural. This is extremely confusing to me. @mizinamo ?
A perfect example is a sentence later in the lessons: "This mustard is hot." where Dieser Senf ist scharf. is an acceptable answer. Here, based on the rule given here, it's not clear that the rule mentioned applies, as dieser is used because Senf is masculine. But, if the above Cinderella rule applies... it would only be accepted if in context with previously applied information, such as Cinderella's carriage is Mustard. In this case, presuming that this mustard is full of mustard seeds, like many of the wonderful German mustards I love so much. Or to be clear if the sentence was "This is a mustard seed." and I replied Dieses Senfkorn ist scharf.*
Very confusing.... does it have something to do with die zwiebel being feminine, and confusion with Diese; because my understanding would be that if plural, it should be Dies Zwiebeln sind scharf. I'm hope this can get cleared up for me, so much time was spent on the rules for using the dies, dieser, diese, dieses forms.
Another example: singular using Dies https://www.duolingo.com/skill/de/Money/2