In a small nonstick skillet coated with cooking spray, saute onion in butter until tender; set aside.
In a large bowl, dissolve yeast in warm water. Stir in honey; let stand for 5 minutes. Add the egg, Parmesan cheese, poppy seeds, salt, rye flour, 1/2 cup bread flour and onion mixture; beat on medium speed until blended. Stir in enough remaining bread flour to form a firm dough.
Turn onto a lightly floured surface; knead until smooth and elastic, about 6-8 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 1 hour.
Punch dough down. Turn onto a lightly floured surface; shape into a loaf. Place in a greased 9x5-in. loaf pan. With a sharp knife, make three or four shallow slashes across top of loaf. Cover and let rise until doubled, about 30 minutes.
In a small bowl, beat egg white with water; brush over loaf. Sprinkle with additional kosher salt. Bake at 350° for 35-40 minutes or until lightly browned. Cool for 10 minutes before removing from pan to a wire rack to cool completely.