I checked the bulbs under the soil and they are maybe 2 inches around. Is that as big as they will get now that they are going to seed or should I cut the top seed bulb off? I don't know what kind they are but do know they are not shallots or leeks. And since the bulb is so large, pretty sure they aren't just green onions. But if they are large onions, why are they going to seed already? Please help.
Pull or cut the flowers off unless you want to use the open flower for culinary purposes then let them open and remove them. Do not let them go to seed.
Continue to fertilize the onions once a month. The safest to use is just dissolve some fertilizer that can be used for foliar applications into some warm water, dissolve it and pour it around the plants evenly. Otherwise you would do what we call “side dress” a dry fertilizer in a narrow band about two to four inches from the plants in a line parallel to the row. Do not get dry fertilizer too close to the plants or you can damage them.
Do not harvest until the tops of the plants fall over. That is your clue to go ahead and harvest them over the following week or so. They will also benefit from a surface layer of mulch. This can be shredded newspaper (black and white only) around the plants.