Spread harissa and preserved lemon sauce, if using, on one half of bread.  Top with potatoes, tuna, hard boiled eggs, pickles, and olives.  Drizzle with tahini. 

Tip: to prepare the fillings, bring a saucepan of water to a boil, add a small unpeeled whole potato and boil for 20 minutes until just starting to soften. Add the eggs to the water and boil for an additional 8 minutes.  Drain and transfer to ice batch. Once cool peel and cube potato and peel and slice eggs.