Heat the butter in a large saucepan.

Add the leeks or onion, and cook over medium-low heat for 5 to 10 minutes, until the onion is tender.

Add the stock, increase the heat to high, and bring to a boil.

Add the peas and cook for 3 to 5 minutes, until the peas are tender.

Add mint leaves.

Turn off the heat and add the salt and pepper.

Puree the soup in a blender or with a immersion hand blender. (Make sure you allow heat out as to avoid the soup exploding.)

Whisk in the creme fraiche and chives and taste for seasoning.