Heat the butter or olive oil in a large saucepan over medium heat.
Add the chopped onion and cook until soft and translucent.
Stir in the crushed garlic and cook for 1 minute.
Add the peas and pour in the vegetable stock.
Bring to a gentle boil and simmer until the peas are tender and bright green.
Add the fresh mint leaves and remove from the heat.
Blend until completely smooth using a hand blender or countertop blender.
Return the soup to low heat and stir in the cream or crème fraîche.
Add the lemon juice and season with salt and black pepper.
Warm gently without boiling.
Serve hot or chilled with a swirl of cream and fresh mint.