This tip is especially useful if you find yourself using half of an avocado when preparing food for yourself. I usually like to cut up half an avocado for salads at lunchtime, and unfortunately, the other half sometimes starts to turn brown, even by the next day and even when wrapped tightly with plastic wrap. This happens because the Omega-3 fatty acids in avocados are sensitive to light, air, and heat, making them oxidize when exposed to either of these.

Instead of wrapping them in plastic wrap, The Kitchn recommends doing this: place your leftover avocado half in a clean cottage cheese or similarly sized opaque container with a piece of cut up onion. Cover the container with the lid and refrigerate. The sulfuric acids from the onion may help to preserve the avocado, but the smell/taste doesn’t transfer to the avocado. Dana from The Kitchn says that avocado halves have stayed fresh for several days using this method!

Thanks to The Kitchn for the tip!