Preheat the oven to 180°C (350°F).
Place ghee in a tagine or oven-safe dish. Add eggplant, onions, tomatoes, zucchini, and potatoes. Season the vegetables with salt and black pepper.
Season the lamb with salt and black pepper, then place it over the vegetables in the tagine.
For the sauce: In a bowl, mix tomato paste, pomegranate molasses, ghee, water, salt, and black pepper until well combined.
Pour the sauce over the meat and vegetables in the tagine. Cover with parchment paper, then aluminum foil.
Bake in the oven for about 2 hours until fully cooked.
In a separate bowl, knead flour, salt, and water to form a soft, smooth dough.
Remove the tagine from the oven, take off the foil and parchment paper, and spread the dough over the top. Brush with beaten egg and sprinkle with nigella seeds.
Return the tagine to the oven under the broiler until the dough is golden and cooked.
Serve the meat and vegetable tagine hot.