I buy a big bag of onions every few weeks and chop them all when I have some time. It takes about fifteen to twenty minutes (I'm kinda slow with the knife) and I have prepared onions ready to grab out of the freezer when a dish calls for them, saving me five minutes while cooking. After you chop each onion place it in a quart size freezer bag and label. Now when I go to make tortilla soup this week I don't have to stop and cut up an onion.
When I make brown rice I will make four to six cups and freeze two cup portions. To reheat, place in microwave safe bowl, add a bit of water and steam in the microwave covered.