Place a large, non-stick sauté pan on a medium-high heat with the oil and garlic. Gently fry for 4 minutes, stirring often, until soft, golden and aromatic. Remove 3 tablespoons of oil, along with half the garlic, and transfer to a small bowl with the harissa, chilli, preserved lemon and lemon juice. Stir together and set aside.
Return the pan to a high heat and add the courgettes and 1¼ teaspoons of salt. Cook for 18 minutes, stirring often, until the courgettes are very soft, but are still mostly holding their shape. Stir through half the basil and transfer to a platter. Spoon the harissa mixture over the courgettes. Leave to sit for 15 minutes, then sprinkle with a pinch of salt and finish with the remaining basil.
