Cook the frozen peas and scallion in the boiling water with the stock concentrate, to taste, or stock cube until tender and cooked through. Drain and discard the scallion, and put the peas into a blender.*
Add the diced mozzarella to the peas in the blender. Liquidize the soup until smooth.
*When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.