Place the brussels sprouts on a large rimmed baking sheet and toss with olive oil and salt to coat. Spread evenly so no Brussels sprouts are overlapping. Roast for 25 to 30 minutes, stirring every 10 minutes, until the sprouts are dark brown and crispy.
Meanwhile, stir the balsamic, honey, and mustard together in a small pot over high heat. Bring to a boil, then reduce to medium-low and cook until reduced by about half (you should have about 3 tablespoons of glaze), 15 to 20 minutes.
Transfer the roasted Brussels sprouts to a large bowl, pour the glaze over top and toss to combine. Garnish with green onions and Parmesan.
