Fold a 36x18-inch piece of heavy foil in half to make an 18-inch square. Place cauliflower, broccoli raab, carrots, and lemon slices in center of foil. Sprinkle with lemon-pepper and salt. Drizzle with the water. Bring up two opposite edges of foil; seal with a double fold. Fold remaining edges to completely enclose vegetables, leaving space for steam to build.
For a charcoal grill, grill packet on the rack of an uncovered grill directly over medium coals for 25 to 30 minutes or until vegetables are tender, turning packet occasionally. (For a gas grill, preheat grill. Reduce heat to medium. Place packet on a grill rack over heat. Cover and grill as directed.)