To make the aioli; mix the mayonnaise grated garlic clove, lemon zest, dijon mustard and salt together and set aside until you need it.
Heat 2 tbsp olive oil in a 10-inch nonstick frying pan over a medium-high heat until shimmering. Add the onion and cook until very softened and translucent. Transfer the onion and oil to a large bowl. Add the eggs and then stir to combine. Fold in crisps until they are well coated.
Wipe out the pan you cooked the onions in and add the rest of the olive oil. Return to medium heat and add egg/crisp mixture and cook, stirring with a rubber spatula, for 10 seconds. Shake pan to evenly distribute mixture, then even out top with spatula.
Cook, shaking the pan occasionally, until tortilla is mostly set but still wet on top, about 2 minutes.
Place a plate over the tortilla. Carefully invert the entire tortilla so that it's resting on top of the plate. Slide the tortilla back into the skillet so that the cooked side is now facing up. Use a spatula to tuck down the edges and create an even shape. Continue to cook, shaking occasionally, until the tortilla is golden brown on the second side and barely cooked through, about 2 mins longer. Slide the tortilla out onto a cutting board.
Cut into squares or wedges and serve immediately with the aioli, or allow to cool and serve at room temperature.