Sauté chopped onion in butter in a medium saucepan until tender; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add 1 cup milk; cook over medium heat, stirring constantly, until thickened and bubbly. Stir in salt and pepper.
Beat egg yolks until thick and lemon colored. Combine cream sauce, beaten yolks, broccoli, and lemon juice in a medium mixing bowl; set aside.
Combine egg whites (at room temperature) and cream of tartar; beat until stiff but not dry. Gently fold egg whites into broccoli mixture. Spoon into a lightly greased 1 1/2-quart casserole. Place in a pan of hot water; bake at 325° for 40 minutes.