How do i grow an onion in a cup?

Question

I was sitting in my garage, and i noticed that one of our onions had grown. like it grew green onion leafs. i put it on a bowl with water, and i want to know of it will grow more onions, or grow flowers, or if i should just throw it away. It has already started growing roots, and its leafs have gotten longer. please let me know what you think. oh p.s. its a purple onion.

Answer 1

Onions are bulbs, and they will sprout if left on the shelf long enough.  Once they sprout, the onion (bulb) itself loses some of its quality.  

If you plant it, it will not grow a new bulb.  But it may eventually produce flowers.  They're not much to look at and really not worth the bother.

If my store-bought onions begin to sprout I either use them immediately for cooking or throw them out if they've also gotten soft.