Preheat your oven to 400°F (200°C) and line a large baking sheet with parchment paper for easy cleanup.
Trim the ends off the Brussels sprouts and slice them in half vertically.
In a mixing bowl, toss the Brussels sprouts with olive oil, sea salt, black pepper, and garlic powder until evenly coated.
Spread the sprouts in a single layer on the prepared baking sheet and roast for 20-25 minutes, flipping halfway through, until the edges are golden and caramelized.
While the sprouts roast, season the chicken breast with a pinch of salt and pepper, then grill over medium-high heat for 6-7 minutes per side until the internal temperature reaches 165°F.
During the last 5 minutes of roasting the sprouts, add the walnut halves to the baking sheet to lightly toast them until fragrant.
Remove the sprouts and walnuts from the oven and immediately drizzle with the balsamic glaze, tossing gently to combine.
Slice the grilled chicken breast and serve it alongside the warm, glazed Brussels sprouts and toasted walnuts.