Mix all the dry ingredients together in a bowl and pour the boiling water over. Stir well to form a batter, then let it rest for 10 minutes.
Preheat oven to 350°F/175°C. Line a half sheet pan with parchment paper or a silicone baking mat*.
Turn the batter out onto the baking sheet and spread it very thin to cover the entire baking sheet. Try to spread the batter out as evenly as possible.Tip: The thinner the batter, the crispier the crackers! Spread the batter as thin as possible for ultimate crisp.
Place into the oven and bake for 30-50 minutes. For square crackers, remove the baking sheet after 20 minutes and cut into squares, then continue baking.
Start checking the crackers after 30 minutes. The edges may cook faster than the center. Break off any pieces that are done and continue cooking the rest. Finished crackers will be crisp, browned, and break off easily. Tip: I don't recommend baking for longer than 50 minutes since the seeds will start to burn. If some of your crackers still seem undercooked, leave them in the oven turned off to cool. They will continue to dry out in the residual heat.
Let the crackers cool completely before storing in an airtight container.
Notes
1) I use 1/4 tsp salt which makes for very lightly salted crackers - perfect for dipping in salty/flavorful sauces or serving with other salty components in a charcuterie board. If you want to serve the crackers on their own, you can use 1/2 tsp salt.