Preheat oven to 400°. Place Brussels sprouts in a 15x10x1-in. baking pan; toss with oil, salt and pepper. Roast 20 minutes, stirring halfway. Add pear and walnuts to pan. Roast until Brussels sprouts are lightly charred and tender, 15-20 minutes.
Meanwhile, in a small saucepan, bring vinegar to a boil. Reduce heat; simmer, uncovered, until syrupy, 8-10 minutes.
In a large bowl, combine Brussels sprouts mixture, balsamic syrup and rosemary; toss to combine.