Preheat your oven to 400°F (200°C) and line a large rimmed baking sheet with parchment paper for easy cleanup.
Prepare the vegetables by peeling and cubing the butternut squash and sweet potato into 1/2-inch pieces, and trimming and halving the Brussels sprouts.
In a large mixing bowl, toss the prepared vegetables with 0.5 tsp of olive oil, 0.125 tsp of sea salt, and 0.125 tsp of black pepper until evenly coated.
Spread the vegetable mixture in a single layer on the baking sheet, ensuring they are not overcrowded to allow for proper browning.
In a small bowl, whisk together the Dijon mustard, maple syrup, garlic powder, and the remaining 0.5 tsp of olive oil to create the glaze.
Season the chicken breast with the remaining sea salt and black pepper, then brush the maple-Dijon glaze generously over the entire surface of the meat.
Place the chicken onto the baking sheet with the vegetables and roast for 20-25 minutes, or until the chicken reaches an internal temperature of 165°F and the vegetables are fork-tender.
Remove from the oven and let the chicken rest for 5 minutes before slicing to keep it juicy, then serve alongside the roasted vegetable medley.