Preheat oven to 400 degrees and spray a roasting pan with cooking spray.
Spread the Brussels sprouts, sweet potato, and onion out in the bottom of the pan. Drizzle the vegetables with olive oil, add garlic and seasonings, and toss to coat.
Place the pork roast on top of the veggies, fatty side down. In a separate small bowl, combine olive oil, minced garlic, rosemary, sea salt, and black pepper, then spread evenly across the top of your roast.
Cook your pork roast and veggies at 400 degrees for 20 minutes. Remove pan from oven and move the pork temporarily to a separate plate, stir veggies, and replace pork on top of the veggies.
Continue roasting at 400 degrees for another 15 minutes, then remove pork and stir veggies again. Sprinkle veggies with ¼ cup shredded Parmesan. Replace the pork roast on top of the vegetables, and top it with ¼ cup shredded Parmesan.
Continue roasting for 30-40 more minutes, or until cheese is browned and pork reaches an internal temperature of 145 degrees. Let pork sit for 10 minutes before slicing.
Notes
Cut your sweet potatoes larger than I did here -- while the cooking time was perfect for the sprouts to get thoroughly caramelized and charred, the smaller cubes of sweet potato didn't hold up as well. The pork takes longer to cook through when sitting on top of the veggies than it does straight on the pan, but that helps keep it lovely and juicy. Cooking times may vary depending on the thickness of your pork roast, so check the internal temperature regularly after adding the Parmesan.