Melt 2/3 cup butter in large kettle. Add onions and cook over medium high heat 8 to 10 minutes or just until tender. Stir frequently. Add flour, stirring until onions are coated. Reduce heat to medium and cook 2 minutes. Stir constantly.
Gradually add beef broth and half and half. Cook, stirring constantly, until thickened and bubbly. Add next 4 ingredients. Remove from heat and stir until cheese melts. Spoon onion mixture into a greased 13 x 9 inch baking dish. Mix bread crumbs and melted butter. Mix well.
Cut 8 strips (1 1/2 inches wide) of wax paper. Space 4 strips diagonally across surface of casserole; sprinkle breadcrumb mixture in spaces between wax paper strips. Remove wax paper. Bake at 375 for 20 to 25 minutes or until thoroughly heated. Before serving, sprinkle parsley on alternating rows, using remaining 4 wax paper strips as guides to cover bread crumbs rows.
Source: My Old Recipes