CHOW gives advice on how to prevent an avocado from browning. You might have noticed that an avocado, when it's cut, starts browning pretty rapidly. The best thing to prevent it from browning is red onion. Cut up a big red onion into chunks and put them in a bowl. Put the cut avocado in the bowl on top of the onions and cover. This works better than plastic wrap or lemon juice or any other method to keep the avocado from browning.