The first thing to look for in chopped onions is whether they have mold or are still firm. If they are mushy, they are already spoiled. If they are brown or discolored, they are also rotten. To determine if an onion is terrible, you can check its surface for black or brown spots. If the spots are mushy, you can cut them off to salvage some good parts. Once cut, onions should be stored in an airtight container in a cool place. Moisture makes onions spoil quickly. Please place them in a cool, dry area, preferably in a refrigerator.