Melt the butter in a saucepan over medium heat. Add the onion and sauté for 3 to 4 minutes or until tender. Add the broth and bring to a boil. Place the peas, mint, salt, and sugar in a blender. (Because hot liquids expand when blended, hold the lid firmly in place with a kitchen towel before blending.) Blend until smooth. Pour the hot broth mixture over them. Return to saucepan to heat through. Serve warm.