To cut a leek, trim the bottom root bunch off and cut the darkest part of the greens off the top, keeping the light green part. To clean the leek, slice it down the middle and rinse water through it to wash away any soil resting inside.
Warm a large saucepan over medium heat. Once it's warm, add the olive oil.
Add the leeks to the saucepan and cook for 5 to 10 minutes, or until they soften, stirring occasionally.
Add the stock or water to the saucepan and bring the stock to a low boil, about 5 minutes or so.
Add the peas to the saucepan and cook the peas for just a few minutes. Frozen peas won't take long to cook. If you're using fresh peas, give them a few more minutes to soften.
Turn the heat off and add the lemon juice and salt to taste.
Puree the soup in batches, adding about ¼ of the soup in a blender at a time. This helps you slowly puree the soup to the thickness that you like and allows you to add or remove liquid to adjust the consistency.
Serve with croutons, lemon wedges, or your favorite toppings.