Sauté onion in butter until tender and almost translucent.
Add garlic and stir, cooking for a minute or less.
Add broth, peas, and mint leaves and cook until peas are tender.
Taste and adjust seasonings and mint to taste.
Carefully transfer to a blender*, food processor, or use an immersion blender, and puree the soup.
Add the heavy cream and blend.
Serve hot, or chill for a few hours and serve cold.
Garnish with a dollop of sour cream and more black pepper.