Fold a 36x18-inch piece of heavy foil in half to make an 18-inch square. Place cauliflower, broccoli rabe, carrots, and lemon slices in center of foil. Sprinkle with lemon-pepper seasoning and salt; drizzle with the water. Bring up two opposite edges of foil; seal with a double fold. Fold remaining edges to enclose vegetables, leaving space for steam to build.
Grill vegetable packet, uncovered, over medium heat 25 to 30 minutes or until vegetables are tender, turning packet occasionally.