In a large bowl combine the water, whole wheat flour, all-purpose flour, sugar, yeast, and salt. Stir until combined in a smooth, stiff dough. Add 1-2 tablespoons of water if the dough is dry.
Knead in the flax and sesame seeds.
Cover the mixing bowl with a clean tea towel and place in a warm location free from drafts. Allow the dough to rise for 1 ½ hours.
Prepare two large sheets of parchment paper. Divide the dough in half and roll each piece into a rectangle on the parchment approximately 14x9" and ⅛" thick. If the dough fights back and won't roll out, allow it to rest another 10 minutes and try again.
Spritz the dough with water and sprinkle with ¼ cup of the topping seeds. Lay another piece of parchment on top and press the seeds into the dough with the rolling pin.
Carefully flip the dough over and remove the parchment. Spritz with water and sprinkle with another ¼ cup of the seed mixture. Lay the parchment back on the dough and press the seeds into the dough using a rolling pin. Repeat with the remaining piece of dough.
Prick the dough all over with a fork. Cut into rectangles or whatever size you desire and lightly separate the pieces. Sprinkle with crushed sea salt if desired and allow the crackers to rise for about 45 minutes. They will get a little puffy.
In the meantime, preheat oven to 350°F. Position the oven racks in the middle.
Bake the crackers for 20 minutes (rotate the pans after 10 minutes) or until the crackers are medium brown in color. Turn off the oven and wait 15 minutes. Prop open the oven door a couple of inches. Leave the crackers in the oven and allow them to cool completely.
When cooled, break the crackers apart and store in an airtight container.