Onion shoots are super easy to grow from your left over onions - brown, shallots or green onions. Even better, they grow quickly, are super hardy and reward you with a permanent supply of green shoots. In many ways, they are easier to grow than lettuce. (And unlike all those posts of regrown carrots, you can actually eat these beauties.)
Regrowing onions is so simple, that beyond a couple of pics, there isn't much to say really. I prefer to grow mine in a little dirt, but many people have success growing them in water.
Keep your spring onion or onion roots and when you are ready, cut them a few centimetres from the bottom.
Fill your container with soil. This one from Alphie Foodie uses a Pyrex Dish - something I am going to do next time as I have a few that are now lidless.
Poke your roots in the dirt.
Water regularly - don't over water - and you will see your new leaves start to sprout in a few days. Wait around 2 weeks at least before you start clipping. It will take a few weeks to get as big as those in the first pic. Make sure they have a little sunlight. (They aren't mushrooms!)
Garlic and ginger are also super easy to grow from produce you already have.