Whisk together apple cider vinegar, olive oil, soy sauce, maple syrup, whole grain mustard and pepper until well combined.
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 preheat oven to 375 degrees F.
Heat 1 tbsp olive oil over high heat in a cast iron pan (or other oven safe skillet) until hot and shimmering
Remove pork tenderloin from the marinade (do NOT throw extra marinade away), shaking off any excess liquid and carefully place in heated skillet. Sear for approximately 1 minute on all 4 sides until golden brown.
While pork it searing, mix together brussels sprouts, onion chunks and apple slices and pour over marinade, stiring to coat.
Once pork is finsihed searing, 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 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 the 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 to rest for approximately 5-8 minutes. Using a slotted spoon, transfer cooked veggies and apples to a serving dish and cover with foil to keep warm (if desired).
While your pork is resting, add a knob of butter (about ½ 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 3-5 minutes (basically the time needed to rest the pork) until it has thickened up to your desired consistancy.
Slice the pork and serve with roast veggies and, if desired, a side of your choice (I recommend sweet potatoes mashed with a little butter, sour cream, salt, pepper and pinch of cinnamon), pouring the pan sauce over the top of everything. Enjoy!