Preheat the oven to 300ºF (160ºC.) Adjust the oven rack to the center of the oven. Prepare two sheets of parchment paper 12 x 17-inches (30 x 43cm.)
In a medium bowl, mix the seeds, furikake, scallions, garlic, chile flakes, then add the water and stir until everything is well-incorporated, Let sit 3 to 5 minutes for the mixture to jell.
Place one sheet of parchment paper on a countertop. Scrape the cracker mixture over the sheet of parchment paper. (It'll seem a little liquidy, but that's okay.) Spread the mixture relatively evenly over the parchment paper with a thin metal icing spatula (best), or the back of the soup spoon (fine to use.) Place the second sheet of parchment over the cracker mixture and use a rolling pin to smooth and even out the cracker mixture. It should reach almost to the edges of the paper but not quite.
Carefully slide the two layers of parchment paper with the cracker mixture between it onto the baking sheet. Peel off the top layer, smooth out any cracks or boo-boos, sprinkle with flaky sea salt, and bake the cracker mixture until it's no longer wet in the center, about 45 minutes. If it still feels damp in the center, you can flip the entire sheet of crackers over, carefully peel off the parchment paper, and bake for another 5 to 10 minutes. Remove from oven and let cool on the baking sheet, then break into shards.