Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper.
Prepare the vegetables by slicing the yellow onion into thin half-moons, halving the Brussels sprouts, and cubing the red potatoes into 1-inch pieces.
In a large mixing bowl, combine the onion, Brussels sprouts, and potatoes with the olive oil, minced garlic, dried thyme, sea salt, and black pepper, tossing until evenly coated.
Place the pork tenderloin in the center of the prepared baking sheet and arrange the vegetable mixture in a single layer around the meat.
Drizzle the balsamic vinegar over the pork and the onions specifically to help them caramelize during the roasting process.
Roast in the oven for 20 to 25 minutes, or until the pork reaches an internal temperature of 145°F and the vegetables are golden and tender.
Remove from the oven and let the pork rest for 5 minutes before slicing into medallions to serve alongside the roasted vegetables.