Stir the first 6 ingredients in a bowl to form a dough. Divide into 4 equal portions. Working with one portion at a time, roll into a ball and place on a sheet of plastic wrap. Top with a second sheet. Roll or press into a 5 inch circle. Place tostada, still in plastic, in the refrigerator. Repeat with remaining dough. Chill tostadas 30 minutes.
Heat a nonstick skillet lightly sprayed with cooking spray over medium heat. Working with 1 tostada at a time, peel off top layer of plastic. Invert into pan, and peel away top sheet of plastic. Cook 45 seconds per side or until set and beginning to brown. Transfer to a wire rack to cool. Continue with remaining tostadas, lightly spraying the pan with cooking spray between each one.
Top each tostada with 2 Tbsp guacamole and 3 Tbsp black beans. Top evenly with sliced radish and top with queso fresco, if desired.