Can you freeze an onion

Yes, you can. The problem with this is as it freezes, the liquid inside the cells will rupture the cell membranes and when you thaw it out again, you wind up with an onion flavored mush. The best thing to do is stew them till they are "broke down" and freeze the onions, stew and all.

Vegetables that are similar in cellular structure like peppers, celery and leafy greens such as collards, spinach, turnip greens and kale will not hold up to being frozen as the same thing will happen.