Process the flax seeds in a clean coffee grinder or blender until they are coarsely chopped (not ground to a meal-like consistency). Transfer to a small bowl and stir in the warm water. Set aside for 45 minutes; the mixture will become pasty.

Meanwhile, cut three sheets of kitchen parchment to fit your dehydrator trays or sheets (if your dehydrator is round with a hole in the center, cut two round sheets with holes in the center and one whole, rectangular sheet).

Combine the whole-wheat flour, rye flour, sesame seeds, sunflower seeds, salt, and baking powder in a mixing bowl. Stir with a wooden spoon until thoroughly mixed. Add the cold water, oil, and the soaked flax seed; stir until very well mixed.

Scoop half of the dough onto a sheet of parchment, then pat into a rectangle (for a round dehydrator tray, spoon portions of the dough around the circle of parchment). Top with another sheet of parchment and use a rolling pin to roll out very thinly; it should be about the thickness of sunflower seeds. Remove the top sheet of parchment, then use a thin knife to score the dough into squares, triangles, or diamonds, whatever size you prefer; 1 ½ inches is a good average size. Transfer the sheet to the dehydrator tray. Repeat with remaining dough.

Dehydrate until crisp and dry, 5 to 6 hours at 140°F. Break along the score lines. Remove the parchment and return the separated crackers to the trays, turning the crackers over. Dehydrate for about an hour longer.