Clean leeks by slicing in half lengthwise, then cutting crosswise. Put in a colander set in a big bowl of cool water. Swish leeks, then change the water to remove sand and dirt. Clean a soft-shell crab by holding the crab in one hand and cutting off the front of the face, about 1/4 inch behind the eyes and mouth, using a pair of kitchen shears. Squeeze out the contents of the sack located directly behind the cut you just made. Lift a pointed end of the crab's outer shell; with fingers or scissors remove and discard the gills. Repeat the process for the other side. Turn the crab over and snip off the small flap called the apron. Pat dry. If pizza dough is in plastic from the grocery store or freezer, take it out and move it to an oiled mixing bowl. Cover the bowl and set in a warm place for at least 30 minutes. If you're working with homemade pizza dough made the same day, you'll be starting this step after the first 1 to 1 1/2-hour rise time. When assembling the pizza, it is a good idea to lay out a thin layer of semolina flour underneath it. This will reduce the chances of the pizza sticking to the assembly surface, and will make the transfer from the surface to the oven very easy.