This punchy, subtly spicy sauce combines North African flavors. Briny green olives and lemon add texture and bright contrast to spicy harissa. Use it as a spread in sandwiches, as an embellishment for a frittata, as a flourish for roasted vegetables or as a dipping sauce (think whole roasted artichokes). For a tangier sauce with a more cooling quality, use whole-milk Greek yogurt in place of the mayonnaise.