Should I  feed people onions on purpose to prove my point in the past?
That depends. Why do you have to prove your point about something as trivial as onions?
You won't know if onions are good for health if you don't try it.
Proving your point can be most satisfying to see happen. Forcing people to eat onions or something they dislike to make them believe your point seems to be irrational, so just feed them what they want so they think you are a sane person. Only crazy people prove points in this way, regardless of health reasons.
OK thanks for the advice. I will think about it.
Think about it a lot. You can make people hate a food if you force them to eat when they don't want to. They can also start refusing to eat anything you cook.