Place oven rack in middle position and preheat oven to 425 degrees.
Pat pork dry with a paper towel and sprinkle with 3/4 teaspoon salt and 3/4 teaspoon black pepper.
Wrap bacon slices around tenderloins. Start on underneath side and wrap around the pork. Continue wrapping bacon slices until tenderloin is mostly covered and bacon seams end up on the underneath side.
Place pork in a non stick skillet, bacon seams down, and cook over medium heat until bacon is golden brown. Flip the pork, as needed, to cook bacon all the way around. This should take about 4 minutes on each side.
Transfer the pork to a rimmed baking sheet and roast at 425 degrees until the pork reaches 140 degrees with a meat thermometer. This will take about 15-20 minutes.
Place apple slices in remaining grease in skillet that pork was seared in and cook over medium heat until they are caramelized. Flip apple slices to cook on both sides. This will take about 2 minutes per side.
Stir in onion slices, rosemary and remaining 3/4 teaspoons salt and 1/2 teaspoon black pepper. and cook about 4 minutes or until onion is tender.
Stir in apple cider vinegar and maple syrup and cook until the liquid is cooked out. About 1 minute.
Slice tenderloin and serve with apple and rosemary mixture on top.