Heat saucepan, add butter & allow to melt. Add the flour & stir well with a whisk for about a minute to allow the flour to cook. Stir in veg broth powder & add the reserved cooking liquid from the cauliflower. Keep stirring until you have a smooth & thickened sauce. Stir in 1/2 of the grated cheese until incorporated, then add the remaining green onion. Turn off heat.