Start by setting a rack in the middle of your oven and heating it to 350°F (175°C). Getting the oven fully preheated before the dough goes in helps the crackers bake evenly from edge to center. It only takes a few minutes, so use this time to measure out your nuts and seeds.
Add the almonds, hazelnuts, and pumpkin seeds to a food processor fitted with an S-blade and pulse until you get a fine, flour-like meal. I usually toss in half of the sunflower seeds here too if I want a smoother texture. If you don't own a food processor, a strong blender works just as well.
Tip the ground nut mixture into a large bowl along with the remaining sunflower seeds, flax seeds, chia seeds, psyllium husk, flaxseed meal, and salt. Stir everything together well before adding any liquid, so the binders are evenly distributed and don't clump in one spot once the water goes in.
Pour in the warm water and mix everything together using a spoon or your hands, not a food processor. The psyllium and chia start gelling almost instantly, so hand-mixing gives you far more control. If the dough feels dry, add water a tablespoon at a time until it holds together.
Divide the mixture roughly in half. Place one portion between two sheets of parchment paper and roll it out into a thin, even layer. If a hole appears, just pinch off a little dough from elsewhere and patch it before moving on.
Peel back the top sheet of parchment and use a knife tip to score the dough into whatever shapes you like — squares, triangles, or diamonds all work well. Scoring now means the crackers snap apart cleanly later instead of shattering unevenly.
Slide the dough, still on its parchment, onto a baking sheet and bake for about 30 minutes, flipping halfway through so both sides crisp up evenly. Keep an eye on the edges near the end, since thinner spots can brown faster than the center.
Transfer the baked sheet onto a wire cooling rack rather than leaving it on the pan. Air circulating underneath stops steam from softening the bottom. Once it's fully cool, snap the crackers apart along your scored lines and they're ready to eat.