Preheat oven to 400°F. Toss together Brussels sprouts, sweet potato, onion, 1 tablespoon olive oil, 1 teaspoon rosemary, ¼ teaspoon salt, and 1/8 teaspoon pepper in a large bowl. Spread into a single layer on a large rimmed baking sheet.
Bake 15 minutes. Meanwhile, combine 4 teaspoons olive oil, the balsamic vinegar, honey, mustard, garlic, 1 teaspoon rosemary, ¼ teaspoon salt, and 1/8 teaspoon pepper. Rub mixture evenly over pork.
Remove pan from oven, and move vegetables away from the center of the pan, and place the pork in the center.
Roast 20 to 25 minutes or until a meat thermometer inserted into the center of the pork registers 145°F.
Let pork stand 5 minutes before slicing.