Prepare charcoal grill with hot coals or heat gas grill to high.
Slice red potatoes crosswise into 1/8-inch-thick coins. Place in medium-size glass bowl, cover with plastic wrap and microwave on high for 5 minutes. Meanwhile, place chicken breasts in 1/3 cup of the dressing, turn to coat and let sit 15 minutes at room temperature.
Add onion slices to potatoes and toss with remaining 2/3 cup dressing. Sprinkle with 1/4 teaspoon of the salt and 1/4 teaspoon of the pepper. Cut four 10-inch-long pieces of heavy-duty foil. Generously coat centers of foil pieces with cooking spray. Divide potato mixture among foil pieces. Create a packet, tightly sealing top edges. Place on grill; cook 10 minutes. Flip over packets; cook 10 minutes more.
Remove chicken from dressing, shaking off excess. Sprinkle with remaining salt and pepper. Grill on high heat about 3 to 5 minutes per side, or until done. Serve chicken and potatoes with sugar snap peas, if desired. Makes 4 servings.