This is a very good and comprehensive companion planting guide from Permaculture Australia. To see/print the thing in a readable format, the PDF address is below. http://www.permaculture.org.au/resources_files/Poster_GDN_Com_Plant.pdf —
"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."