Preheat oven to 350 degrees F (175 degrees C).

Heat bacon grease in a large skillet over medium heat. Add onions; cook and stir until onions are translucent, about 5 minutes.

Arrange sauerkraut and apples in a large roasting pan. Top with cooked onions and remaining bacon grease.

Mix chicken broth, brown sugar, and mustard powder together in a microwave-safe bowl. Microwave on high until sugar dissolves and broth is heated through, about 2 minutes. Stir and pour over sauerkraut mixture, mixing to combine.

Cut several deep slits in the top of the pork loin and place in the center of the sauerkraut mixture. Cover roasting pan with aluminum foil.

Roast pork in the preheated oven until heated through, about 1 hour. Reduce oven temperature to 250 degrees F (121 degrees C). Continue roasting pork until tender and pulls apart easily, 2 to 3 hours. Remove from oven and cool until easily handled, about 15 minutes.

Discard pork bones, shred pork using 2 forks and return to pan with sauerkraut mixture. Stir well to combine.