Roughly chop the leeks, potatoes, carrot and garlic.
Heat the oil in a large saucepan and gently sauté the leek and garlic for about 10 minutes .
When the leek has softened, add the carrot and potato pieces. Put the lid on and sauté for 5 minutes.
Add the stock and bay leaf. Bring to the boil, then simmer for 20 minutes, until the potatoes have started to fall apart.
Add the peas and cook for 5more minutes, then turn off the heat and add the mint leaves. If you want a rustic texture, mash it with a potato masher. If you want a smooth soup, use a blender and whiz it until silky.
Add the lemon juice just before serving.