Wash ribs thoroughly. Soak in a large bowl of water, changing the water every 30 minutes, to remove excess fat, about 2 hours. Drain and pat dry with paper towels. Mix lemon juice, molasses, vinegar, mustard, ketchup, cinnamon, curry powder, cumin, salt, and pepper in a large, deep bowl. Combine onion, tomatoes, sweet peppers, parsley, garlic, and cayenne pepper in a food processor; pulse to form a paste. Stir onion paste into the lemon juice mixture in the bowl. Add lamb ribs; turn to coat. Cover with plastic wrap. Marinate in the refrigerator, at least 10 hours. Preheat oven to 300 degrees F (150 degrees C). Line a large baking sheet with aluminum foil and set a rack on top. Arrange ribs on the rack. Bake ribs in the preheated oven until fat has rendered and meat is tender, 1 1/2 to 2 hours. Cover ribs with aluminum foil and let rest for 5 minutes before serving.