Preheat your oven to 400°F and line a large rimmed baking sheet with parchment paper for easy cleanup.
Trim the woody ends off the Brussels sprouts and slice them in half, then cut the chicken breast into uniform 1-inch cubes.
In a large mixing bowl, combine the sprouts and chicken cubes with olive oil, sea salt, black pepper, and garlic powder, tossing until every piece is well-coated.
Spread the mixture in a single layer on the baking sheet, ensuring there is enough space between the sprouts so they roast and crisp rather than steam.
Roast for 20 to 25 minutes, tossing the ingredients halfway through, until the chicken is cooked through and the sprout leaves are beautifully charred.
While the tray is in the oven, add the balsamic vinegar to a small saucepan over low heat and simmer for 3 to 5 minutes until it reduces into a thick, syrupy glaze.
Transfer the roasted chicken and sprouts to a plate, sprinkle with chopped walnuts for crunch, and finish with a generous drizzle of the warm balsamic glaze.