White onion/Red onion/Sweet onion
The same technique works well for every variety of bulb onion, I've found. It's a bit more involved, but still easy! There's two ways to grow from a bulb.
FROM THE CENTRE
Start by cutting around the edges of the onion, from top down, similar to if you were cutting up an apple or a pepper. Be careful not to damage the centre of the onion—where the sprouts reach down to the root base. It’ll look like a little white tubular structure right at the centre of the onion.
You have two options now:
You can put the onion base in water. Make sure it’s just the base, where the roots are! Then, switch out the water daily. When the roots are about an inch long, plant in soil and water immediately. Proceed to water whenever the soil dries out.
Place the onion directly in about an inch of soil. Water immediately. Proceed to water whenever the soil dries out.
And that's it! Soon, you'll have onion scales growing which can be used as a substitute for the actualvonion in any recipes. Eventually, you'll have a whole onion bulb you can use!
FROM THE BASE
1. Cut half an inch to an inch up from the root base.
2. Follow step 2 above!