Should I prune my onions?

Topping your onions apparently ensures larger bulbs. Each stalk rising from an onion represents one layer of the onion bulb. If you cut off your onion stems to 10 cm (4”) from the bottom, it will force the energy into the bulb. A week after pruning the greens, you should see a new stem appear, therefore a new ring on the final product.

The onion green is edible, no need to throw it out! Some reference material available on YouTube:

How To Grow Big Bulb Onions
https://www.youtube.com/watch?v=sn19fbNm_PI

How To Grow Bigger Onions. Does Topping Them Really Work?
https://www.youtube.com/watch?v=zPLmoo4AMSs