In a large bowl, combine 3 cups flour, yeast, sugar and salt. Add potatoes and butter. Beat in warm milk until smooth. Stir in enough remaining flour to form a firm dough. Beat for 2 minutes. Turn onto a lightly floured surface; knead until smooth and elastic, about 5-7 minutes. Place dough in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 45 minutes.
Punch dough down; turn onto a lightly floured surface. Roll into an 18-in. circle. Transfer to a lightly greased 14-in. pizza pan. Sprinkle cheese over center of dough to within 5-in. of edge. Sprinkle with thyme and rosemary. Bring edges of dough to center; twist to form a knot. Cover and let rise until doubled, about 30 minutes.
In a small bowl, combine egg and milk; brush over top. Sprinkle with poppy seeds. Bake at 350° for 40-45 minutes or until golden brown. Cool for 20 minutes before slicing. Serve warm.