She scared the life out of me today...!!!!!

I don't know why I am posting this here but I need to let it out...

We were having dinner... and her dad had made himself a salad with tomatoes cucumber onion etc... she was cruising and playing around... she suddenly came and swiped at her dads salad... there was this huge onion (almost intact) in his salad (1" by 1.5").. I told him this piece is too big for her so give it her so that she will sit quietly... I was worried she would cut herself with fork or something... anyhow I take my eyes off for a sec to pour water and that huge ass chuck of onion goes right in her mouth... I still have no idea how she managed to fit it in her mouth... but I froze... I mean I froze for a few seconds and next thing I know I pick her up, pry her mouth open and pull that stupid piece of onion out... I was so shocked that I left her on the floor sitting and crying for a sec... till her dad picked her up...

After I comforted her and put her to sleep, I burst into tears... I still can't stop crying... I am so scared thinking what may have happened if I hadn't seen her for few more seconds, what If she choked on it... I am so scared on how will I take care of her when I am obviously such a terrible mother... my husband kept telling me that it wasn't my mistake there is no way we could have thought she would put that big onion in her mouth but I can't stop crying... I am so scared !!!!!