Preheat your oven to 425°F. Line a baking sheet with parchment paper.
Trim the tough ends off the Brussels sprouts and cut them in half. Place them in a large bowl.
In a small bowl, whisk together the olive oil, honey, minced garlic, 1 1/2 tablespoons of the balsamic vinegar, salt, pepper, and red pepper flakes (if using) until well blended.
Toss the Brussels sprouts with the marinade until they are evenly coated. Spread them in a single layer on the prepared baking sheet with the cut sides facing down.
Roast for 20 to 25 minutes until tender and slightly crisp. Remove from the oven and drizzle with the remaining 1 1/2 tablespoons of balsamic vinegar. Taste and adjust salt if needed before serving warm.