Combine chopped spinach, breadcrumbs, and onion, and set aside.
Beat egg yolks until thick and lemon colored. Add butter, salt, and pepper. Add spinach mixture; mix well. Set aside.
Beat egg whites (at room temperature) until stiff but not dry. Gradually add spinach mixture to egg whites; fold gently until well blended. Spoon into a lightly greased 8 1/2- x 4 1/2- x 3- inch loaf pan.
Place loaf pan into a 12- x 8- x 2-inch baking dish; pour hot water into dish to a depth of 1 inch. Bake at 350° for 50 minutes or until wooden pick inserted in center comes out clean. Invert spinach loaf onto a serving platter, and garnish with lemon slices and fresh spinach leaves.