Place the okra in a non-metallic bowl, pour over the lemon juice and leave to stand for 10–15 minutes.
Heat the oil in the base of a tagine or a large, heavy-based saucepan over a medium heat, stir in the onion, garlic, chillies, coriander seeds and sugar and cook for 2–3 minutes until the onion starts to soften.
Add the okra and lemon juice and stir in the tomatoes, then cover and cook gently for 20 minutes until the okra is cooked through but still retaining its crunch. Season to taste with salt and pepper. Garnish with the preserved lemon and serve with couscous, if liked.