Preparation Melt butter in a large heavy pot over medium heat. Add onion and cook, stirring often, until softened but not browned, 6-8 minutes. Add 2 cups broth and bring to a boil. Add peas, reduce heat, and simmer gently until tender, about 5 minutes for fresh peas, about 2 minutes for frozen. Remove pot from heat. Add parsley, mint, and remaining 2 cups broth to pot. Purée soup in a blender or with an immersion blender, thinning with water if soup is too thick, until smooth. Season soup with salt and pepper. Whisk crème fraîche and cream in a small bowl to blend. Serve warm soup topped with chives, passing crème fraîche mixture alongside for spooning over.
Preparation Melt butter in a large heavy pot over medium heat. Add onion and cook, stirring often, until softened but not browned, 6-8 minutes. Add 2 cups broth and bring to a boil. Add peas, reduce heat, and simmer gently until tender, about 5 minutes for fresh peas, about 2 minutes for frozen. Remove pot from heat. Add parsley, mint, and remaining 2 cups broth to pot. Purée soup in a blender or with an immersion blender, thinning with water if soup is too thick, until smooth. Season soup with salt and pepper. Whisk crème fraîche and cream in a small bowl to blend. Serve warm soup topped with chives, passing crème fraîche mixture alongside for spooning over.