Preheat the oven to 250 degrees F (120 degrees C). Line a baking sheet with aluminum foil.

Mix brown sugar, barbecue seasoning, paprika, sea salt, pepper, and garlic powder together in a bowl.

Coat both sides of ribs liberally with mustard and then with seasoning mixture. Place ribs onto the prepared baking sheet and cover with additional foil.

Bake in the preheated oven until meat pulls away easily from the bone, about 2 hours. An instant-read thermometer inserted into the center should read 145 degrees F (63 degrees C). Remove from the oven, discard foil, and cover ribs in barbecue sauce. Bake, uncovered, until barbecue sauce begins to caramelize, about 15 minutes. Let rest for 10 minutes before serving.