Cook the potatoes in boiling water until tender. Heat 2 tablespoons of olive oil in a pan and saute the onion until lightly browned. Stir in garlic and cook until fragrant. Drain the potatoes and return to the saucepan. Over low heat, mash the potatoes, then stir in onions and garlic. Remove from heat. In a mixing bowl, whisk eggs with Harissa, herbs and salt. Gradually beat in the egg mixture into the potatoes. Heat remaining oil in heavy frying pan, add egg and potato mixture and cook over a very low heat until the bottom is set. Brown the top under a hot broiler.