1In 3-quart saucepan, melt butter over medium heat. Cook onions in butter, stirring frequently, until tender. Add peas and broth; heat to simmering. Cook 10 minutes, stirring occasionally. Remove from heat; cool 20 minutes.
2In blender, place pea mixture, buttermilk, chopped mint and salt. Cover; blend on medium-high speed about 1 minute, stopping frequently to scrape sides, until smooth. Pour into large bowl. Cover; refrigerate about 1 hour or until well chilled. Garnish individual servings with crème fraîche, mint leaves and pepper.