Cook green beans according to package directions; drain well.
Cook onion and parsley in 2 tablespoons butter in a large skillet over medium-high heat, stirring constantly, until tender. Reduce heat to medium-low. Stir in flour, salt and pepper. Add sour cream, stirring well. Add green beans; cook until thoroughly heated, stirring occasionally.
Spoon mixture into an ungreased 13x9" baking dish; sprinkle with cheese. Combine breadcrumbs and remaining butter; sprinkle over cheese. Bake at 325º for 25 minutes or until cheese melts and breadcrumbs are browned.