Whether you prefer white, yellow or red onions, storage tips for cut onions are the same. Onions have a smell that tends to invade other items in the refrigerator, including milk and potatoes, so there are some special steps to take to avoid this problem. When storing cut onions in the refrigerator, it is best to use them within seven days.
Place your cut onions in a plastic or glass storage container with a tight-fitting lid. Avoid metal containers as they can cause the onion to have a metallic taste. The plastic or glass storage container reduces the chances of the onion odor seeping into other items in your refrigerator.
Insert the cut onions in a sealable plastic bag and then place the bag inside a second sealable bag. Remove excess air from the bags before sealing them. The air will cause the onions to dry out and lose nutrients at a quicker pace. The purpose of storing the onions in two sealable bags is to prevent the odor from escaping and ruining other items in your refrigerator.
Spread butter over an onion that was cut in half. Spread the butter on the side that you exposed to the air and do not remove the remaining skin. Once you have spread the butter onto the freshly cut surface, place the onion with the cut side down, onto aluminum foil. Wrap the foil around the onion. Place the onion in a sealable bag.