Heat the oil in a heavy-based pan. Add the onion, cook slowly for 4-5 minutes without colouring, stirring occasionally. Add the peas.
Pour the stock or water into the pan and allow simmer for 10-15 minutes until the peas are completely tender but still retaining their colour and all of the flavours have had time to infuse. Add the mint.
Transfer the soup to a food processor or liquidiser or use a hand blender and whiz until blended.
To serve, stir the cream into the soup and allow warm through. Season to taste and then ladle into warmed bowls. Top with fresh mint and crème fraiche.