Peel the onion and cut it into small cubes. Cut off the white part of the leek, wash and finely chop.
In a saucepan, melt the butter over medium heat and add the chopped vegetables. Sauté for about 5 minutes until the vegetables are soft. Stir diligently.
Add the frozen peas, increase the heat and sauté for another 5 minutes, stirring frequently.
Pour the broth over the peas, add salt, pepper, and sugar. Cover the pot and bring the soup to a boil. Once it begins to boil, reduce the heat to a minimum.
Add both heavy cream and sour cream. Stir and simmer gently for a minute. Now turn off the stove, you won't need it anymore, and let's finish the soup.
Add fresh mint leaves to the soup pot.
Thoroughly mix the soup with an immersion blender. Be patient and careful; blend for 5 minutes to get the most delicate possible cream.
Using a spoon, strain the soup through a sieve 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!