Prepare the pasta frolla and knead in the nuts and fruit, being careful not to overmix the dough. Divide the dough into 4 pieces and roll each into an even cylinder about 12 inches long. Place the cylinders on 2 jelly roll pans or cookie sheets that have been lined with parchment. For the egg wash, whisk the egg and salt together and paint the tops of the logs of dough, using a soft brush.
Bake the logs at 350 degrees for about 30 minutes, until they are well colored and firm to the touch. Remove from the pans or sheets to racks to cool. When cool, slice through the logs diagonally at ½-inch intervals, using a sharp, serrated knife. If the biscotti seem underdone in the center, or you wish to toast them lightly, arrange them on parchment-lined pans or cookie sheets, cut side down, and bake them at 350 degrees for 15 minutes (this is an optional step and should not be necessary).
Store the mosaici in a tin between layers of wax paper.