In a large bowl, combine 1 1/2 cups flour, yeast, sugar and salt. Add warm water, mustard and butter; stir well. Stir in the remaining flour, 1/2 cup at a time, beating well after each addition. When the dough has pulled together, turn it out onto a lightly floured surface and knead until smooth and elastic, about 6 minutes. Cover and let rest for 10 minutes.
On lightly floured surface, roll dough to a 10x15 rectangle. Layer corned beef, Swiss cheese, and sauerkraut in the center third of the rectangle. Use a knife to make cuts from filling to dough edge, approximately 1 inches apart. Folds strips across bread, alternating sides. Cover loaf and let rise in a warm area for 45 minutes.
Preheat oven to 375 degrees F (190 degrees C).
Brush risen loaf with beaten egg. Bake in preheated oven for 30 minutes, until golden brown.
