Preheat your oven to 425°F (220°C). Peel the butternut squash, remove seeds, and cut it into roughly 1-inch cubes. Slice the red onion into wedges. This size ensures even roasting and caramelization.
In a medium bowl, whisk together the Dijon mustard, pure maple syrup, olive oil, garlic powder, onion powder, salt, and pepper until fully combined. This glaze will infuse the chicken and veggies with a wonderful balance of sweet and tangy flavors.
Place the chicken breasts in the bowl with the glaze and turn to coat each piece thoroughly. In a separate bowl, toss the butternut squash and red onion with dried thyme, rosemary, a pinch of salt, pepper, and a drizzle of olive oil.
Line your sheet pan with parchment paper or foil if desired. Spread the squash and onion evenly across the pan, creating a bed for the chicken. Place the glazed chicken breasts on top, ensuring they aren’t touching to allow even cooking.
Roast in the preheated oven for 25-30 minutes or until the chicken reaches an internal temperature of 165°F (74°C) and the squash is tender with caramelized edges. Halfway through cooking, flip the chicken and toss the veggies to ensure even browning.
Remove the sheet pan from the oven and let the chicken rest for 5 minutes before slicing. This helps keep the meat juicy. Serve the chicken breasts alongside the roasted butternut squash and onions for a colorful, nutritious plate.