How to Make It
In a blender or food processor, puree the onion. Strain the puree through a double layer of cheesecloth or a fine-meshed strainer into a deep baking dish; you should have about 1/2 cup of juice. Stir in the milk, olive oil and 1 teaspoon of pepper. Add the lamb and coat with the marinade. cover and refrigerate for 24 hours, turning the racks from time to time.
Scoop the yogurt into a fine stainless-steel strainer set over a bowl and let drain overnight in the refrigerator.
Light a grill. When the fire is hot, grill the eggplants and green pepper, turning often, until they are charred on all sides and very soft, about 10 minutes for the pepper and 30 minutes for the eggplants. Transfer the charred vegetables to a plate and let cool. Scrape the charred skins off the eggplants and pepper. Core and seed the pepper.
In a food processor or blender, puree the eggplants and pepper with the garlic. Add the drained yogurt, butter and lemon juice and process until smooth. Transfer the sauce to a saucepan and simmer over moderate heat for 2 minutes. Season with salt and pepper and set aside.
Drain the lamb, reserving the marinade. Generously season the racks with salt. Grill the lamb over a hot fire, starting bony side down and turning with tongs as needed; baste the racks with the marinade during the first 15 minutes of cooking and rotate them, moving them away from the coals if the meat starts to burn. If the tips of the bones begin to char, wrap them in foil. Stand the racks on their ends and sear until browned. Total cooking time will be about 30 minutes, depending on the size of the racks; medium-rare meat will measure 125° to 130° on an instant-read thermometer. Remove the racks from the grill and let rest for 10 minutes before carving.
To serve, reheat the eggplant sauce and spread it on plates or a platter. Carve the racks into chops, cutting down between the ribs. Arrange the chops on the sauce and serve at once.