For seasoning mixture, combine thyme, paprika, garlic salt, and pepper; set aside. Fold a 36x18-inch piece of heavy foil in half to make an 18x18-inch square. Cut potatoes crosswise into 1/4-inch-thick slices. Place potato slices and onion slices in the center of the foil. Drizzle with oil. Sprinkle with seasoning mixture.
Bring up opposite edges of foil and seal with a double fold. Fold remaining edges to completely enclose the vegetables, leaving space for steam to build.
Grill on the rack of an uncovered grill directly over medium heat for 20 to 25 minutes or until potatoes are tender. Makes 4 servings.