Set your oven to 400°F (200°C) so it's fully heated by the time you're ready to roast. Line a large baking sheet with parchment paper to make cleanup painless later. A hot oven from the start helps the vegetables caramelize instead of just steaming through.
In a mixing bowl, combine the apple cider, Dijon mustard, olive oil, garlic, rosemary, thyme, salt, and pepper. Whisk until it looks smooth and well blended. Set aside about a third of this mixture to brush on later, since you don't want to reuse marinade that touched raw pork.
Pat the tenderloin dry with paper towels first, since a dry surface helps the marinade stick and browns better in the oven. Coat it evenly in the remaining marinade and let it sit for at least 20 minutes. If your schedule allows, 2 hours in the fridge gives noticeably deeper flavor.
While the pork marinates, spread the potatoes, carrots, Brussels sprouts, and onion wedges across your lined baking sheet. Drizzle with olive oil, then sprinkle on the garlic powder, paprika, salt, and pepper. Toss everything by hand until each piece is coated.
Slide the pan into the oven and roast the vegetables alone for about 15 minutes. Potatoes and carrots simply need more time than the pork does. This head start means everything reaches perfect doneness at the same moment.
Push the vegetables toward the edges of the pan to make room in the center. Place the marinated tenderloin in that space and brush it with your reserved marinade. Roast for another 20 to 25 minutes, checking with a meat thermometer until it reads 145°F (63°C).
Pull the pan out and let the pork rest for 5 to 10 minutes before cutting into it. This short rest lets the juices settle back into the meat instead of spilling out onto the cutting board. Slice the pork and serve it right alongside the roasted vegetables, spooning any pan juices over the top.