Preheat the oven to 425℉. Line a sheet pan with parchment paper.
Place the pork tenderloin in the center of the sheet pan. Set aside.
In a medium bowl, combine the brussels sprouts, onions (if using), maple syrup, olive oil and rosemary. Toss well. Spoon the brussels sprouts around the pork. Sprinkle the brussels with garlic powder, salt and pepper.
Roast the pork and brussels sprouts 25 to 30 minutes, or until the center of the pork has reached 145℉. Slice the pork and serve with the roasted brussels sprouts.