Preheat your oven to 400°F and line a large baking sheet with parchment paper for easy cleanup.
Trim the ends off the Brussels sprouts and cut them in half lengthwise.
Toss the halved sprouts on the baking sheet with half of the olive oil, half of the sea salt, and half of the black pepper.
Spread the sprouts in a single layer and roast for 20-25 minutes, tossing halfway through, until the edges are dark and caramelized.
While the sprouts roast, season the chicken breast with garlic powder and the remaining salt and pepper.
Heat the remaining olive oil in a skillet over medium-high heat and sear the chicken for 6-7 minutes per side until cooked through.
In a small bowl, whisk together the balsamic vinegar and honey until well combined to create a thick glaze.
Toast the walnuts in a small dry skillet over medium heat for 2-3 minutes, stirring constantly until they are fragrant.
Remove the sprouts from the oven, toss them immediately with the balsamic glaze and toasted walnuts, and serve alongside the sliced chicken.