This is another question where it's easy to use the fruit/vegetable in the singular, i.e. "I like slicing onion", which is perfectly correct English, but the software does not accept this.
I LOVE the Indonesian course, particularly because it's really good at picking up lots of variations in the English answers, but this here is a definite fail.
So if you can add these variant correct answers, somebody, it'd be wonderful!
In Indonesian, plurals aren't always necessary. Bawang can mean both “onion” and “onions.” In fact, after a number for example, like dua buah bawang, you are not allowed to reduplicate the word. Personally, I've only ever heard “slicing onions” in the plural in all my life, but sapere_aude too seems to think the singular is valid, so I guess report it; the more accepted variations, the merrier!