In a 12-inch skillet cook onion in hot butter over medium heat about 15 minutes or until very tender and browned, turning heat down if onions start to overbrown. Stir in balsamic vinegar, brown sugar, sage, tomato paste, salt, and pepper. Add beans; stir to coat.
Cover and cook over medium heat for 10 to 15 minutes or until heated through, stirring occasionally. Transfer to a serving bowl. Top with creme fraiche and tomatoes, if desired. Makes 12 to 14 servings.