Step 1In a blender or coffee grinder, grind the sunflower and pumpkin seeds into a fine powder. Be careful not to blend too long or they will turn into seed butter. Separately, grind the flax seeds into a powder. (The sesame seeds do not need to be ground.)
Step 2In a mixing bowl or in the bowl of a stand mixer, combine the seeds, flour, salt, honey and vegetable oil with three-fourths cup water. Mix by hand or in a stand mixer with a paddle attachment to form a firm ball of dough. It should not be sticky.
Step 3On a lightly-floured surface, knead the dough by hand for about 30 seconds to make sure all the ingredients are evenly distributed and that the dough holds together. It should be slightly tacky but not sticky. Add more whole-wheat flour or water, if needed.
Step 4Heat the oven to 300 degrees. Line three baking pans with parchment or a silicone baking liner. Divide the dough into three equal pieces. Set two of the pieces aside and roll out the remaining piece on a floured surface with a floured rolling pin. Roll it to less than one-eighth-inch thick, or nearly paper thin. Continually lift the dough as you roll it so that it doesn't stick to the surface; dust more flour under it if need be. You can also flip the dough over and continue rolling with the bottom side up. If the dough resists, lift it gently and set it aside and begin rolling out one of the other pieces. You can return to the first piece after a few minutes and it will roll more easily. Continue rolling out all three pieces in this manner.
Step 5Use a knife or pizza cutter to cut the rolled dough into rectangular or diamond-shaped crackers. (You can also use a small biscuit cutter, dipped in flour, to make round crackers.) The crackers do not need to be the same size. Carefully transfer the cut crackers to the baking pans. The pieces can be nearly touching, as the crackers will not spread or rise. Fill each pan as full as it will allow.
Step 6Combine the egg with one-half cup water. Brush the egg wash lightly over each of the crackers and top with sesame seeds.
Step 7If your oven has enough shelves to accommodate all the pans, you can bake them all at once. If not, bake them in shifts. After placing the pans in the oven, bake for 10 minutes. Rotate the pans and bake for another 10 minutes. Make one final rotation and continue baking until they are done. (The thinner you roll them the faster they will bake -- they typically take about 25 to 30 minutes total, but this can vary from oven to oven.) The crackers will be a rich golden brown and should be fairly dry and crisp when they come out of the oven.
Step 8Leave them on the pans to cool; they will crisp up even more. If they do not snap cleanly after they cool, return the pan to the hot oven for a few more minutes. The crackers can be stored in an airtight container or resealable plastic bag and kept for at least 8 days at room temperature or indefinitely in the freezer.