1. Preheat the oven to 425°F (220°C). Combine 2 tbsp lemon juice, 1 tbsp oil, the mint, and harissa in a zippered plastic bag. Add the lamb and turn to coat. Refrigerate for at least 1 hour and up to 24 hours.

2. Meanwhile, place the eggplants on a baking sheet and prick with a fork several times. Bake for 30 minutes or until the skins have blackened. Let cool until easy to handle. Remove and discard the skins. Drain the eggplant flesh in a colander for 15 minutes. Purèe the eggplant, yogurt, remaining 3 tbsp lemon juice, tahini, and garlic in a blender. Season with salt and pepper.

3. Remove the lamb from the marinade. Season with salt. Heat the remaining 2 tbsp olive oil in a large ovenproof frying pan over high heat. Add the lamb and cook, turning occasionally, about 10 minutes, until browned. Transfer to the oven and roast about 10 minutes, until an instant-read thermometer inserted in the center reads 130°F (55°C) for medium-rare. Transfer to a platter and let stand 10 minutes.

4. Carve the lamb and serve on warmed plates with the purèe. Pour the pan juices over, and sprinkle with chopped mint.