In a 5-qt. Dutch oven, melt the butter over medium heat. Add the scallions, leek and potato and sauté for 3 - 5 minutes being careful not to brown the ingredients.
Add the chicken stock and bring to a boil.
Add the shelled peas and reduce the heat to a simmer. Cook for 15 - 20 minutes or until the peas are very tender.
Remove the soup from the heat and let stand for 20 - 30 minutes.
Add the lemon juice, cream, mint and salt and pepper to taste.
Puree with an immersion blender or process in batches using a blender or food processor until smooth and creamy. Add more cream or broth if the soup is too thick.
If the soup needs to be a little warmer, reheat at a low temperature.
Ladle into warmed bowls and garnish with a dollop of sour cream and a sprig of fresh mint.