The bulb needs the foliage to develop properly :-) If you cut off the foliage, the bulb will not mature. If you want greens for your salads, grow scallions. These mature very quickly compared to bulb onions, do not form a real bulb (although if left in the ground for the whole season they will develop a smallish bulb) and are what are commonly sold in the grocery as "green" or spring onions. They should be available in nurseries now as seeds, sets or starts.