In a 12-inch skillet cook onion in hot butter over medium heat about 15 minutes or until very tender and browned, stirring occasionally. Stir in maple syrup, vinegar, brown sugar, 2 tablespoons 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 sour cream. If desired, top with tomatoes and sage leaves.