Growing Green Onions in a Mason Jar

I saw a pin with a tip for growing green onions in a mason jar for a never ending supply. It said to put the left over white/root part in a mason jar with some water and place it in the window. This is supposed to make the onion grow again, and you can then trim off the green whenever you need some. I tried it. I used some green onion in dinner and didn't cut off any of the white. I put it in my jar, added enough water to cover it, and placed it in the window. Well one week went by and nothing happened. I changed out the water for clean and waited some more. Another week went by and nothing still. If anything, a little mold was growing on it. My family got sick of it and it was thrown out, even before I got a picture. This one did not work for me. If it has worked for you, and I did something wrong, I welcome your tips!