Cut that leaf the bud is on and eat it. Eat the bud too.
When the onion plant sends up a seed leaf and it flowers, the bulb development stops and the plants energy goes to the flower to make seed.
Sometimes plants that have sent up a seed leaf will produce a bulb too if the bud is plucked, but it is never as nice a bulb as from those plants that do not flower. The flower leaf comes from the root up through the center of the bulb and makes the center of the bulb open and sometimes soft. These bulbs do not store as well.
Gardening at 5000 feet elevation, zone 4/5 Northern Utah, Frost free from May 25 to September 8 +/-