Whisk together apple cider vinegar, olive oil, soy sauce (or coconut aminos), maple syrup, whole grain mustard and pepper until well combined into a marinade.
Place pork tenderloin in a large freezer bag and pour marinade over it. Let marinate in the fridge for at least 1 hour to overnight.
Remove pork tenderloin from the fridge and place on a paper towel lined plate. *Do not throw away the marinade!*Gently pat dry and allow the meat to sit at room temperature for at least 15-20 minutes while you prep the veggies. Preheat oven to 375 degrees F.Prep the Brussel sprouts, onion and apples and place in a mixing bowl. Pour the leftover marinade on top of the veggies and mix to coat. Set aside. Once the pork tenderloin has come to room temperature heat a couple tablespoons of olive oil over high heat in a cast iron pan (or other oven safe skillet) until hot and shimmering.Season all sides of the pork with a little salt and pepper and then carefully place in the preheated pan.*You should immediately hear a sizzle (if you don't, the pan is not hot enough).*Sear for approximately 1 minute on all 4 sides until golden brown. Once pork is finished searing take the pan off the heat and use a slotted spoon to transfer veggies and apples to the skillet.*Note* You can move the pork around so that it sits on top of the veggies or is nestled in the middle, your choice.
Take the remaining marinade (from the bowl of veggies) and pour over the top of the pork tenderloin.
Place the skillet in the preheated oven and roast at 375 degrees F until veggies are tender and pork reaches an internal temperature of 145 degrees F, around 30-40 minutes.*Note* Because of varying sizes of pork tenderloins the best way to ensure your meat is properly cooked is to use a quick read meat thermometer. Remove skillet from the oven and use tongs to place pork on a cutting board or plate. Loosely cover the tenderloin with aluminum foil and allow it to rest for approximately 5-8 minutes.Then using a slotted spoon (so that you do not pick up the juices from the pan), transfer cooked veggies and apples to a serving dish and cover with foil to keep warm.
While your pork is resting, add a knob of butter (about ½-1 tbsp) to the remaining juices in the pan and bring to a boil over medium-high heat on the stove.
Simmer and reduce the sauce for around 3-4 minutes until it has thickened up to your desired consistency.
Slice the pork and serve with the roasted veggies and apples and, if desired, a side of your choice (I recommend mashed sweet potatoes), pouring the pan sauce over the top of everything. Enjoy!