In a large bowl, dissolve yeast in 1/2 cup warm water. Add the salt, remaining water and 2 cups flour. Beat until smooth. Add enough remaining flour to form a firm dough. ( I mix this in my KitchenAid with dough hook.
Turn onto a floured surface; knead until smooth and elastic, about 6 minutes. Place in a greased bowl, turning once to grease top. Cover and let rise in a warm place until doubled, about 50 minutes.
In a large skillet, cook sausage over medium heat until no longer pink; remove with a slotted spoon and set aside. In the drippings, saute peppers and onion until tender; drain.
Press half of the dough onto the bottom and 1/2 in. up the sides of a greased 15-in. x 10-in. x 1-in. baking pan. Spread sausage evenly over the crust. Top with peppers and onion. Sprinkle with mozzarella cheese. Roll out remaining dough to fit pan; place over cheese and seal the edges.
In a small bowl, beat egg and water. Stir in the remaining ingredients. Brush over dough. Cut slits in top. Bake at 400° for 20-25 minutes or until golden brown. Cut into squares.