Heat butter in a pan over low to medium heat until foaming.
Fry chopped shallot and garlic for 8 to 10 minutes until softened.
Stir in the cucumber and nutmeg and season with salt and white pepper.
Fry for 3 to 4 minutes until the mixture is soft and translucent.
Add 220g peas and 200ml stock and bring to a simmer.
Cover and cook gently for 6 to 8 minutes until peas are tender.
Remove from heat and allow the mixture to cool slightly.
Blend the mixture in a food processor to a smooth purée.
Add lettuce and sorrel to the processor and blend again until smooth.
Pass the soup through a fine sieve into a bowl.
Whisk in crème fraîche and chill in the fridge until cold.
Ladle into bowls and garnish with prawns if desired.