In a medium stock pot, melt butter over medium heat.
Add chopped onion and cook until onion is softened but not brown, 6-8 minutes, stirring occasionally.
Add stock and bring to boil. Add peas, turn down heat and simmer 3-4 minutes, until just tender. Remove stock pot from heat and add parsley and mint. Purée with immersion blender, thinning with water if too thick.
Season with salt and pepper.
Refrigerate and serve chilled.
Garnish with sour cream and fresh mint leaves.