Prep charcoal grill with medium-hot coals or heat gas grill to medium-high.
Using 12-inch-wide heavy-duty aluminum foil, cut 4 pieces 32 inches long; fold each piece in half to make a 16 x 12-inch rectangle. Generously coat foil pieces with nonstick cooking spray.
Grate zest from lemon; place in a medium-size bowl. Cut a thin slice from the top and bottom. Cut off peel, removing all white pith. Holding lemon over the bowl with the zest, carefully cut along both sides of each section to free it from the membrane. Discard seeds. Let sections fall into bowl. Using two forks, break up sections into bits.
Stir in mustard, oil, garlic, salt and pepper. Add chicken, zucchini and onion; toss to coat. Let sit 5 minutes.
Meanwhile, place potatoes in a glass pie plate with 1/4 cup water. Cover and microwave on HIGH for 5 minutes. Carefully drain off water; stir potatoes into chicken mixture. Evenly place mixture on one half of each foil piece.
Fold second half of foil over chicken and vegetables. Tightly seal sides and front edges, leaving space for steam to build. Place on grill rack; grill 25 minutes. Carefully open packets; transfer contents to 4 bowls.