Peel 1 medium yellow onion and finely chop it. Wash 4 ounces leek stem (white part) and finely chop it.
In a saucepan, melt ½ stick unsalted butter over medium heat and add the chopped vegetables. Sauté for about 5 minutes until the vegetables are soft, stirring diligently.
Throw in 3 cups frozen peas, increase the heat, and sauté for another 5 minutes, stirring frequently.
Pour 4 cups chicken broth over the peas, then add 1 teaspoon salt, ⅛ teaspoon ground black pepper, and ½ teaspoon granulated sugar Cover the saucepan and bring the soup to a boil. Once it begins to boil, reduce the heat to a minimum.
Add ⅔ cup heavy cream and 1 Tablespoon sour cream, stirring to combine. Simmer gently for a minute. Turn off the stove, as you won't need it anymore, and let's finish the soup.
Add 10 leaves fresh mint to the pot of soup.
Thoroughly mix the soup with an immersion blender. Be patient and careful, blending for about 5 minutes to achieve the most delicate cream possible.
Using a spoon, press the soup through a strainer into a clean pot. This part is more tedious than it sounds; it will take a while to push the soup through the leftover pea pods. You should end up with about two tablespoons of dry leftovers in the strainer.
Season the soup with salt and heat if necessary. And we are in the final step!