Make the spice rub: Combine the peppercorns, coriander seeds, fennel seeds and cumin seeds in a small dry skillet over medium-high heat; cook, stirring, until toasted, about 2 minutes. Transfer to a spice grinder and grind to a fine powder. In a small bowl, mix the freshly ground spices with the paprika, cayenne and 1 teaspoon salt.
Preheat a grill to medium high and prepare for indirect grilling: For a charcoal grill, bank the hot coals to one side and leave the other half empty. For a gas grill, preheat all the burners, then turn off half the burners and keep the others on medium-high heat.
Grill the lamb: Coat the lamb with the olive oil and sprinkle with the spice mixture. Grill over direct heat, turning as needed, until browned all over, 6 to 8 minutes. Transfer to indirect heat (the unlit side of the grill); cover and cook until an instant-read thermometer inserted into the center of the lamb registers 125 degrees F for medium-rare doneness, 20 to 24 minutes. Transfer the lamb to a cutting board and let rest 8 minutes.
Meanwhile, make the yogurt sauce: In a small bowl, mix the labne, olive oil, herbs, Aleppo pepper, lemon zest and 2 teaspoons lemon juice. Season with salt, black pepper and more lemon juice, if desired. Carve the lamb racks and serve with the yogurt sauce.
Photograph by Con Poulos