Preheat your smoker to 225ºF with the lid closed. If you're using a Traeger, use the Super Smoke setting.
While the grill preheats, rub the mustard over the chicken before adding the seasoning blend. Massage or stir to evenly coat the chicken in the seasoning and set aside.
Add the thinly sliced onion to the bottom of a cast iron skillet or smoker safe dish. Place the chicken thighs on top of the onion before going on the grill.
Cook for 1 hour.
Pour the BBQ sauce over the chicken and cover the skillet with foil. Increase the grill temperature to 350ºF and cook for 75-90 minutes until the chicken shreds easily.
Shred the chicken and stir everything together in the pan. Place the tortillas on the grill and let everything cook for 20-30 minutes until the BBQ sauce has thickened and the tortillas are crispy.
Once the tortillas are crispy, add about 3 tablespoons of heated black beans (microwave or heated on the grill) to 8 tortillas along with an ounce of cheese on each. Place another crispy tortilla on top and close the grill for 3-5 minutes to melt the cheese.
Serve the chicken on top of the tostadas and garnish with your choice of salsa and cotija cheese, if desired.