Rinse and pat the thighs dry with paper towels (see notes), and then trim off the excess fat. Season with salt and pepper.
Mix the granulated onion, granulated garlic, thyme, oregano, sage, black pepper, cayenne, mustard powder, paprika, and celery seeds in a small bowl until well combined.
Rub the thighs thoroughly with the seasoning blend until well-coated. Let them rest in the fridge for at least 30 minutes, but overnight is better, to allow the spices to penetrate the meat.
Fill your pellet grill's hopper with your choice of wood pellets. Lightly oil the grates so the chicken doesn't stick, then preheat it to 225°F (107°C). This may take 10-15 minutes.
Place the thighs directly on the grates (no need to bring them to room temperature), then close the lid and smoke them for 1-2 hours, or until the thickest part of the thighs reaches 170-175℉ (75-80℃). Flip them once halfway through smoking if you'd like.
Optionally, brush the thighs with barbecue sauce during the last 10-15 minutes of smoking or after they're done.
Once the chicken thighs are done, take them out of the smoker and transfer them to a plate.
Tent the thighs loosely with aluminum foil and let them rest for 10 minutes so the juices can redistribute for tenderer, juicier chicken.
Serve with your favorite sides, and enjoy!