The next time you have green onions, don't throw away the white ends. Simply submerge them in a glass of water and place them in a sunny window. Your onions will begin to grow almost immediately and can be harvested almost indefinitely. We just use kitchen scissors to cut what we need for meals. I periodically empty out the water, rinse the roots off and give them fresh water.
And if you have leftovers of the actual green parts, chop them up how you would normally use them, dry them completely and toss them into a plastic water bottle and freeze them. Then use as needed!