Rinse (see notes), pat dry, and lightly salt and pepper the chicken. Place the thighs in a large bowl or zip-lock bag.
Whisk the olive oil, mustard, soy sauce, garlic, parsley, thyme, lemon juice, and pepper flakes in a medium mixing bowl.
Pour the marinade over the thighs, mix thoroughly, cover the bowl or seal the zip-lock bag, and marinate in the refrigerator for around 2 hours. You could go overnight so that the flavors have time to infuse into the chicken.
Remove the marinated chicken thighs from the refrigerator and drain them. When smoking, it's not always necessary for them to come to room temperature.
When ready to smoke, remove the chicken from the marinade, drain off the excess, and discard the remaining marinade.
Preheat the smoker to 225-250°F (107-120°C). I used a Traeger pellet grill for this recipe.
Place the chicken thighs directly on the grates, leaving a little space between them for airflow and smoke circulation.
Close the smoker and let the chicken thighs smoke for 1½-2 hours or until the internal temperature reaches 165°F (74°C) on a thermometer inserted into the thickest part of the thighs. Thighs vary in size, resulting in varying smoke times.
During the smoking process, you can baste the chicken thighs during the last 5-10 minutes of smoking.
Once the chicken thighs reach the desired internal temperature, take them out of the smoker and transfer them to a cutting board.
Let the chicken thighs rest for 5-10 minutes to retain their juices and ensure tenderness.
Serve with BBQ sauce and your favorite side dishes.