Preheat your oven rack to a high temperature of 400°F (200°C). Position a large baking sheet, preferably a sturdy, rimmed one, in the center of the oven. For easier cleanup, line the baking sheet with parchment paper or heavy-duty aluminum foil. Thoroughly wash the Brussels sprouts and remove any loose, damaged, or yellow outer leaves. Trim off the hard, woody end of the stem, then cut each sprout in half lengthwise from the tip to the root. If you have particularly large sprouts, quarter them to ensure even cooking and maximum crispiness.

In a very large mixing bowl, combine the halved Brussels sprouts, 3 tablespoons of the olive oil (reserve 1 tablespoon for the glaze), the kosher salt, and the black pepper. If you are using minced garlic, add it to this mixture now. Use your hands or a large sturdy spoon to toss the sprouts vigorously until every single piece is lightly coated with the oil and seasoning. Ensure the sprouts are well distributed.

Carefully pour the seasoned Brussels sprouts onto the prepared baking sheet. Spread the sprouts out into a single, even layer, making sure they are not overcrowded (overcrowding will steam them). Place the baking sheet into the preheated 400°F (200°C) oven and roast for a total of 15 to 20 minutes. After the first 10 minutes of roasting, carefully remove the pan and use a spatula to toss or flip the sprouts to ensure they brown evenly on all sides. Return the pan to the oven.

While the Brussels sprouts are completing their initial roast, prepare the sweet glaze. In a small bowl or a liquid measuring cup, combine the remaining 1 tablespoon of olive oil, the packed light brown sugar, and the 3 tablespoons of balsamic vinegar. Whisk these three ingredients together thoroughly until the brown sugar has mostly dissolved into the liquid mixture and you have a smooth, syrupy consistency. Set the glaze aside.

After the sprouts have roasted for 15 to 20 minutes and the cut sides are deeply golden brown and starting to crisp up, remove the baking sheet from the oven. Pour the prepared balsamic brown sugar glaze directly over the hot sprouts. Use a spatula to gently toss the sprouts on the pan, coating them evenly with the glaze. Return the glazed Brussels sprouts to the 400°F (200°C) oven for an additional 5 to 10 minutes. Watch the sprouts closely during this final stage, as the sugar can burn easily. The goal is to allow the glaze to bubble, thicken, and caramelize onto the sprouts, making them sticky and tender.

Once the sprouts are tender, sticky, deeply caramelized, and have crispy edges, remove the baking sheet from the oven. Transfer the Balsamic Brown Sugar Brussels Sprouts to a serving platter and scrape any remaining sticky glaze from the pan over the top. Serve the dish immediately while it is piping hot.