Preheat the oven to 160c/320f (fan).
Cut each nori sheet in half. Stack the halves and slice into three even rectangles. You will have 24 pieces.
Place onto a lined baking tray. Using a pastry brush, lightly brush the top side of each piece with oil.
Sprinkle with sea salt and sesame seeds if using.
Bake for 7 to 9 minutes, keeping a close eye near the end (to stop them burning, you don't want any golden colour as they will taste bitter).
Remove straight away and let them cool on the tray until crisp.
Store in an airtight container - they will stay crisp for 1 to 2 weeks. If they start to soften or feel stale, re-crisp them in the oven for 2 to 3 minutes and let them cool again before serving. Pop the desiccant packet that comes inside the nori packaging into your container to help keep them fresh