Whisk the lentil flour, ground flaxseed, cumin, smoked paprika, turmeric, garlic powder, and fine sea salt together in a 3-quart / 2.8 L mixing bowl until the color is even.
Add the olive oil and 90 ml warm water and stir to a shaggy dough. Knead in the bowl for 1 minute until it feels like soft play dough. If it's crumbly, add water 1 tsp at a time. Cover and rest for 10 minutes while the oven heats to 180 C / 350 F with racks in the upper and lower thirds.
Divide the dough in two. Roll each half between two sheets of parchment into a rectangle about 28 x 30 cm, 2 mm thick. Peel off the top sheet.
Sprinkle half the sesame seeds and half the flaky salt over each rectangle and press them in with the rolling pin. Slide each parchment sheet onto a 33 x 46 cm / 13 x 18 inch sheet pan. Cut into 5 cm / 2-inch squares with a pizza cutter, cutting all the way through.
Bake for about 22 minutes, swapping the pans between racks and rotating them at the 11-minute mark. The crackers are done when they look dry and matte with golden edges. The thin edges brown first, so if they darken before the middle dries, move them to a rack and bake the rest 2 to 3 minutes longer.
Cool on the pans for 10 minutes. The crackers crisp as they cool. Snap apart any that are still joined and store once completely cool.