A tartine is a slice of bread with toppings—basically, an open-faced sandwich. Here, instead of basic tuna salad sandwiches, we make tartines and give them a Mediterranean spin by spicing things up with harissa, a North African chili sauce and an essential Milk Street pantry ingredient. Olives lend brininess, orange zest brightens the flavors and coriander adds floral notes. In a nod to the classic, we keep the familiar crunch of celery; feel free to swap in fresh fennel if you have it. Instead of stirring mayonnaise into the tuna mixture, we spike mayonnaise with some harissa and citrus juice, then drizzle it over the open-faced sandwiches.