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.
*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.
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