You should never plan to keep a portion of a sliced onion. He says it\'s not even safe if you put it in a zip-lock bag and put it in your refrigerator. It\'s already contaminated enough just by being cut open and out for a bit, that it can be a danger to you (and doubly watch out for those onions you put in your hotdogs at the baseball park!)
Also, dogs should never eat onions. Their stomachs cannot metabolIize them!!!