Preheat oven to 325 degrees F. In skillet cook bacon over medium heat until crisp. Remove; drain on paper towels. Crumble; set aside. Drain skillet of all but 1 tablespoon drippings. Cook onion in drippings over medium heat until tender. Transfer to large bowl. Add beans and all but 2 tablespoons of the bacon; set aside.
In another bowl whisk together sour cream, broth, flour, rosemary, mustard, and pepper. Stir into bean mixture. Transfer to 2-quart casserole. Bake, covered, 45 minutes, stirring once halfway through.
In bowl combine remaining bacon, parsley, and lemon peel; sprinkle on beans just before serving. Makes 10 (3/4-cup) servings.