Heat the oil in a 6-quart or larger pressure cooker. Add the onions and cumin seeds and cook over medium-high heat, stirring frequently, for 1 minute. Stir in the water, lentils, prunes, and lamb. Set the eggplant on top. Sprinkle with the salt and cinnamon.
Lock the lid in place. Over high heat bring to high pressure. Reduce the heat just enough to maintain high pressure and cook for 12 minutes. Turn off the heat. Allow the pressure to come down naturally, about 10 minutes. Remove the lid, tilting it away from you to allow steam to escape.
Skim off any fat that rises to the top. Stir well to blend in the eggplant. Add more cinnamon and salt, if needed. Stir in the lemon zest and enough of the lemon juice to balance the sweetness of the prunes.
To prepare the Minted Yogurt, stir the yogurt and mint together. Serve each portion of stew with a generous dollop of Minted Yogurt on top.
Vegetarian Version: Decrease cumin to 1 teaspoon. Increase water to 3 cups and lentils to 1½ cups. Omit lamb.
After cooking, stir in 1 teaspoon harissa (Moroccan chile paste) or a generous pinch of cayenne.
Use fresh cilantro instead of mint in yogurt topping.
Transformation (Follow basic recipe except as noted.)
Moroccan Chicken and Lentil Stew: Instead of lamb, use 3 to 4 pounds bone-in, skinned chicken parts. Cook for 8 minutes high pressure plus natural pressure release.