Combine water and lentils in large saucepan; bring to boil. Reduce heat and simmer, covered, until lentils are tender, about 15 minutes; drain. (Since I used canned lentils, I skipped this step)
Meanwhile, to make dressing, whisk together lemon juice, oil, 1/4 teaspoon of salt, and 1/8 teaspoon of black pepper in large bowl.
Spray broiler rack with nonstick spray. Preheat broiler.
Toss together lamb, oregano, and remaining 1/4 teaspoon salt and 1/8 teaspoon black pepper in large bowl to coat evenly. Thread lamb and tomatoes evenly onto 4 (12-inch) metal skewers. Broil 5 inches from heat, turning, until lamb is cooked through and tomatoes are softened, about 8 minutes (mine took a total of about 20 minutes to completely cook due to the fact that the lamb chunks were larger in size).
Add lentils, spinach, bell pepper, and onion to dressing; toss until mixed well. Spoon lentil salad onto platter and top with kebabs.