Finely chop the onion and garlic. Heat the oil in a saucepan, then gently sweat the chopped onion and garlic for about 5 minutes, stirring regularly.
Add the peas and stock to the pan. Bring to the boil, then gently simmer for 5 minutes.
Add the spinach and fresh herb, then turn off the heat. Put the lid on and let it rest for a couple of minutes until the spinach wilted. Blitz with a hand-held blender or in liquidiser until smooth and fluffy.
Squeeze in the lemon juice and serve with some mint leaves scattered on top. You can also add a dollop of crème fraîche or Greek yoghurt if you like.
Serve lukewarm.