Heat oven to 350°. Stir cornmeal, sesame and sunflower seeds, oil, flaxseed, and 2 tsp. salt in a bowl. Slowly stir in water until a thick, chunky dough forms. Using a greased spatula, spread dough evenly over the surface of a parchment paper–lined baking sheet. Sprinkle with remaining salt; bake until golden and crisp, about 40 minutes. Let cool; break into pieces to serve.