Cook the sweet potato in a pan of lightly salted boiling water for 15-20 minutes, or until soft when pierced with a fork. Drain well, then mash and set aside.
Place the ground pork in a bowl, add the mashed potato, grated apple, and Cajun seasoning. Grate 1 of the onions and add to the pork mixture with salt and pepper to taste and the chopped cilantro. Mix together, then shape into 4-6 equal-size burgers. Cover and let chill for 1 hour.
Slice the remaining onions. Heat 1 tablespoon of the oil in a skillet. Add the onions and cook over low heat for 10-12 minutes, stirring until soft. Remove the skillet from the heat and set aside. Wrap each burger in 2 slices of bacon.
Preheat the barbecue. Lightly brush the burgers with oil, and cook over hot coals for 5-6 minutes on each side, or until cooked to personal preference. Serve with the fried onions.