This is the easiest DIY ever.
1. Buy green onions for cooking something tasty, like these.
2. Use green parts for recipe.
3. Stick the remaining onion in a glass of water somewhere sunny or bright.
4. Change the water every few days and add more onion as you use your original purchase up.
5. The ones in the glass will grow big and strong. Cut off pieces as needed. The onions will keep regrowing.
That's it. I started these at the beginning of March, and they are going strong. The new shoots taste just as good as the originals and didn't cost me anything. I suppose eventually they will run out of steam, but by then I will have certainly gotten my money's worth!