Preheat the oven to 250 degrees F. Line a sheet pan with foil.
In a small bowl, mix together the brown sugar, paprika, black pepper, garlic powder, onion powder and 1 teaspoon salt.
Lay the ribs onto the prepared sheet pan and season on both sides with salt. With the meat side of the ribs face down, brush with half of the mustard and sprinkle over half of the seasoning mix. Turn the ribs over, so the meat side now faces up, and use the remaining mustard and seasoning mix on the other side. Bake the ribs meat-side up until they are very tender, 2 to 2 1/2 hours.
Remove from the oven and spread the ribs with half of the BBQ sauce. Return the sheet pan to the oven for another 30 minutes, allowing the sauce on top to become sticky and delicious.
Transfer the ribs to a cutting board and allow them to rest for a few minutes before slicing into individual ribs. Serve with the remaining BBQ sauce on the side.