I have already blogged about this topic but have been asked to provide a step-by-step tutorial to show how it is done. Growing spring onions is easy and can be done with the leftover bottoms of spring onions you buy from the supermarket.
Step One: Buy a bunch of spring onions. Make sure they are fresh, the greens are crispy and the roots are attached. If they are wilted or slimy you might not get the result you are after.
Step Two: Cut onions with a clean, sharp knife a few inches from the roots.
Step Three: Place in container, with the roots facing the ground. This could be a jar, plastic bucket, glass jug, basically anything clean that holds water.
Step Four: Add the water. You only need an inch or two to cover the roots. Don’t fill the container or cover the whole plant.
Step Five: Empty the water and add fresh water each day and enjoy watching your spring onions grow.
Under the Mango Tree 
I did this too! I found that after a month the bottoms got wrinkly and hollow-ish and threw them out at that point (I changed the water continuously). Up until that point though, I got lots of fresh onion greens out ot them.
Thanks for this. Looks like the only mistake I made was I didn’t leave any of the green onion. I cut down to the root then sat that in water lol.. I’ll give this a try again and report back. Thanks Mr.CBB
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