Cook onion slices and parsley in 2 tablespoons butter until onion is tender.
Blend in flour, lemon zest, salt and pepper. Stir in milk; heat until thick and bubbly. Add sour cream and beans; heat through. Spoon into an ungreased 2-quart baking dish; sprinkle with cheese. Melt remaining butter and toss with bread crumbs; sprinkle on top of beans. Broil 3 to 4 inches from heat source for 3 minutes, or until golden.