Question

I like onions, They are good for me这一句对吗？

Answer 1

不对，一个句子不能有两个谓语，因此可以改为并列句，即把逗号换为and. 或者改为定语从句，即I like onions which are good for me.

Answer 2

对
I like onions, they are good for me.
我喜欢洋葱，它们对我有益。