In a saucepan, soften the onion in butter. Add the grated courgettes and peas and stir to coat in the butter. Add the stock and bring to the boil until peas are tender. Add mint to the pan together with the crème fraîche. Heat through then blend with a hand held liquidiser.
Chill in the fridge for 1 hour before serving.
If you wish to serve this soup hot, return to the heat after blending to warm through.
Substitute mint for basil, omit the crème fraîche and decorate with crème fraîche and crispy bacon bits for a different take.