The quickest way to proceed is to fill a kettle first and put it on to boil. When it's boiled, measure the amount you need into a pan and put on the stove to come back to a boil.
Add the frozen peas, scallions, salt and lime juice and let everything bubble together for 7 minutes.
Discard the scallions and blitz the peas and their liquid with the pesto in a blender.*
Pour into a thermos flask that you've left filled with hot water and then emptied and make sure you screw the top on securely.
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.
