Onions tend to turn to mush if frozen straight away. They also can release their odor in the freezer, if not packed correctly. Slice and fry them first, to a little browning. Then pack them tightly in as small a jar as possible, with an airtight lid. You can use freezer tape to insure a seal on many jars. You can also roast onions before freezing, but the prior method is easier to pack with less air space. With roasted garlic and fried onions, I don’t trust freezer bags.
Oxygen absorbers come in very handy for a variety of uses. As a “home cook”, I use them to:
Keep whole coffee beans sealed and fresh.
Pack grains, freshly-ground whole wheat flour and cornmeal from getting stale, rancid or insects.
Keep dehydrated foods fresh for much longer.
And, as I have a home-model freeze drier, they get packed with freeze dried foods, so they’ll keep for many years, sealed in mylar.
As in your situation, they may be too much of a twiddle unless one processes or stores a lot of food, which I usually do.
