Onions will stay fresh for ‘weeks’ at these ‘perfect’ conditions

Onions produce ethylene gases, which speeds up the ripening process. And the high moisture content of potatoes is harmful to onions. 

By keeping onions and potatoes together, they both will rot faster than if stored on their own. 

Sliced onions keep best in an airtight container in the fridge for one week. You can store them in the freezer for several months, but the texture will change. That shouldn’t matter too much if you’re cooking the onions, however.