1) To make the dough: Mix and knead together all of the cracker ingredients (except the seeds) to a smooth, fairly stiff dough. Add 1-2 more tablespoons of water if the dough is dry.
2) Knead in the seeds.
3) Let the dough rise, covered, for 60 to 90 minutes, until it's expanded a bit.
4) Divide the dough in half. Working with one piece at a time, roll it into a rectangle approximately 14" x 9", a generous 1/8" thick. This will probably require you to roll the dough until it fights back; give it a 10-minute rest, then come back and roll some more. It may need two rest periods to allow you to roll it thin enough.
5) For easiest handling, turn the dough onto a piece of parchment paper. Spritz the dough with water. Sprinkle with 1/4 of the topping seeds, lay a piece of parchment on top, and press the seeds in with a rolling pin. Turn the dough over, peel off the parchment, and repeat. Set the seeded crackers on a baking sheet, and repeat with the remaining piece of dough.
6) If you don't have parchment, roll on a rolling mat or on a very lightly floured or lightly greased work surface; and transfer the seeded crackers to a lightly greased baking sheet. Sprinkle each sheet of crackers with some sea salt or flavored salt, if desired. Crush the sea salt between your fingers or grind it in a salt mill if it's very coarse.
7) Prick the dough over with a fork, and cut it into rectangles, whatever size you like. Pull the crackers apart just a bit; you don't need to separate them completely. Let the crackers rise for 30 to 45 minutes. while you preheat your oven to 350°F; they'll get just a bit puffy.
8) Bake for 20 minutes, until the crackers are a medium brown. Turn off the heat, wait 15 minutes, then open the oven door a couple of inches and let the crackers cool completely in the turned-off oven. When they're completely cool, break apart, if necessary, and store airtight.
Yield: two 14" x 9" sheets of crackers.