Remove roots, outer leaves, and tops from leeks, leaving 6 inches of each leek. Cut each leek in half lengthwise. Cut each half crosswise into 1-inch-thick slices. Rinse under cold water; drain. Set aside.
Wash brussels sprouts thoroughly under cold running water, and remove discolored leaves. Cut off stem ends, and cut large sprouts in half; set aside.
Coat a large nonstick skillet with cooking spray; add oil. Place over medium heat until hot. Add leeks and garlic; sauté 5 minutes. Add sprouts, carrot, and water; cover and cook 15 minutes. Spoon vegetable mixture into a 1 1/2-quart casserole coated with cooking spray; set aside.
Melt margarine in a small saucepan over medium-low heat; stir in flour. Gradually add milk, stirring with a wire whisk until well blended. Increase heat to medium; cook 6 minutes or until thickened, stirring constantly. Remove from heat; stir in lemon-pepper blend and salt. Pour sauce over vegetable mixture. (Cover and chill mixture now, if desired.)
Combine cornflake crumbs and cheese; stir well. Sprinkle over vegetable mixture. Cover and bake at 350º for 30 minutes. Uncover and bake an additional 20 minutes or until thoroughly heated.