Trim and discard the dark green portion of the leek leaving just the light green and white portions. Do not cut off the roots.

Slice the leek in half lengthwise. Fan the cut halves slightly and hold under cold running tap water to wash away any sand stuck in the layers.

Place one half of the leek on a cutting board cut side down. Beginning just above the root, slice the leek in 1/4-inch strips from the white part all the way to the top of the light green part. Then slice crosswise into a 1/4-inch dice all the way to the root. Discard the root. Repeat with the other half of the leek.

Set a Dutch oven or large pot over medium heat and add the olive oil. When the oil is hot, add the chopped leek, stirring to coat with the oil. Reduce the heat to low and cook the leek gently until softened, about 10-minutes. Do not allow the leek to turn golden or brown.

Add the peas, chicken broth, and mint. Increase the heat and bring to a boil, then reduce to low. Simmer for about 7 to 10 minutes for fresh peas, about 5 to 8 minutes if using frozen peas.

Remove from the heat and purée with an immersion blender or standard kitchen blender.

In a small dish combine the yogurt and the milk stirring to the thickness of heavy cream. Add the mixture to the pea soup and stir to combine.

Season to taste with salt and pepper. Stir in the lemon juice. Return the pot to medium-low and reheat the soup gently.

Ladle the soup into serving bowls and garnish with mint sprigs, croutons and a drizzle of olive oil.