Marinate the Tenderloin - to a large plastic bag or a bowl, add the pork tenderloin, the apple cider vinegar, the honey, onion powder, and garlic powder. If the marinade doesn't cover the top of the pork, add enough water to cover. Allow pork tenderloin to marinate for at least an hour, but up to 8. Drain pork.

Preheat oven to 400 degrees Fahrenheit. In a cast iron skillet, heat the olive oil over medium high heat. Once the pan and oil are hot, add the pork tenderloin, and sear until brown on each side.

Once seared, place the pork tenderloin in a 9x9 inch baking dish. Top with a whole grain mustard and drizzle with honey. Add the sliced onions and apples to either side of the dish. Cover with tin foil and bake for 20-30 minutes.

Remove pork from the oven, and check internal temperatures making sure the pork is at least 150 degrees Fahrenheit.

Allow pork to cool for 5 minutes before slicing. Drizzle with any juices at the bottom of the dish, and serve with onions and apples.