Similar to leeks, these vegetables will re-grow from the white root end. Cut the stalks off as you normally would, and place the root end in a shallow bowl of water – enough to cover the roots but not the top of your cutting. Place it in a sunny window position, occasionally spraying your cutting with water to keep the top moist.
After a few days, you should start to see roots and new leaves appear. After a week or so, transplant it into soil with just the leaves showing above the level of the soil. The plant will continue to grow, and within a few weeks it will sprout a whole new head.
Alternatively you can plant your cutting directly into soil (without starting the process in water) but you will need to keep the soil very moist for the first week until the new shoots start to appear.