Preheat the oven to 300 degrees F and place racks in the upper-middle and lower-middle positions. Have two half sheet pans ready.
Combine half of the white and black sesame seeds with the bonito flakes, sugar, wasabi powder, togarashi, and salt in a spice grinder. Pulse until coarsely ground, about three pulses. Transfer the seasoning mix to a small bowl and mix in the remaining white and black sesame seeds.
Place a sheet of nori on a cutting board and, using a pastry brush, lightly moisten with the water. Immediately sprinkle 1 teaspoon of the seasoning mix evenly over the nori. It may start to curl up on the edges; that’s okay. Using a large chef’s knife, slice each sheet into 6 even rectangles and transfer to the half sheet pan. Repeat with the remaining nori sheets, dividing them evenly between the two pans. Leave at room temperature for 30 minutes to dry. (You will have extra seasoning; save for another use, like…popcorn.)
Add sesame oil to a spritzer and evenly spritz the seasoned side of the nori sheets (You will have extra oil). Transfer to the oven and bake for 5 minutes. Rotate the pans and continue to bake until the snacks are crisp but not yet brown, about 3 more minutes.
Transfer the nori sheets to a wire rack to cool. To store, transfer to a gallon-size zip-top bag and add the desiccant packet from the nori packaging to help maintain crispness. The seaweed snacks will keep for at least 1 week at room temperature.
I like a hot air popcorn popper for toasting the seeds, though you will have to hold a small sieve over the pop chamber to keep the seeds from flying out. In my machine, toasting usually takes about 45 seconds.