Cut lamb into 4 cm pieces. Mix 2 tablespoons olive oil with the juice from the 1/2 lemon and pour over the lamb. Mix well, then sprinkle with salt and pepper. Arrange some onion slices and bay leaves on top. Cover and refrigerate for 5 to 6 hours.
Cut eggplants, red pepper, green pepper, tomatoes, and onions into slices. Thread lamb pieces alternately on 4 large skewers with the vegetable slices and bay leaves. Brush grill grate with olive oil. Grill over medium-high heat, turning frequently, for 10 minutes until cooked through. Serve immediately while hot.