Cut the ends off green onions and plant in potting soil. Of course. You can even cut them a little higher and just place them in water and they will grow back! You can have oodles of green onions at your disposal without having to have a garden.
This article titled "How To Grow Sprouted Onions" at Alternative Gardening talks about exactly that. I had no idea. All this time, if I didn't eat an onion in time, I would simply plant the whole thing as-is in the soil.
Growing Green Onions, and Things I'm Thinking About (4-8-2014
Save your large egg shells next time you bake, wash them out, plant them with seeds. Once the plants are large enough, just transfer the whole egg into the ground. The shell is actually a great fertilizer and the roots will be able to break right through.