How to store a raw, cut onion
Unlike whole onions, cut onions are best stored in the fridge in a sealed plastic bag or air-tight container.
It’s important to note, fruits such as apples and pears, celery and eggs will absorb onion odours if stored near them, so ensure onions are properly sealed to prevent this from happening. In addition, onions can also absorb other odours.
When stored this way, they will last seven to 10 days.
How to store cooked onions
Cooked onions should also be stored in an airtight container and placed in the fridge. Like most leftovers, cooked onions will last up to four days in the fridge.
source: express.co.uk