In a small bowl, stir together paprika, brown sugar, chili powder, cumin, garlic salt and salt. Set aside.
Prepare outdoor grill with medium-hot coals, or heat gas grill to medium-high. Cut each potato lengthwise but not all of the way through. Score flesh of the potato crosswise with a paring knife, in a criss-cross fashion. Place 1/4 of the onions, 1 tablespoon butter and 1/2 teaspoon of the spice rub in the center of each potato. Press halves together; seal tightly in a double layer of foil. Place foil packets on grill over indirect heat; close lid. Cook for 45 minutes to 60 minutes until tender.
Remove skin from thighs. Coat both sides of chicken in remaining rub. Place chicken in a 13 x 9-inch microwave-safe baking dish. Cover with plastic wrap and vent in one corner. Microwave on high for 10 minutes.
Grill thighs for 3 minutes per side or until internal temperature registers 170 degrees F on an instant-read thermometer. Serve chicken thighs with grilled potatoes.