Preheat oven to 400 degrees F.
Cut the butternut squash into small cubes (approximately ½ inch). Next, wash the brussels sprouts and chop the bottom ends off, then cut each sprout in half. Don't worry if some of the outer leaves fall off, as they tend to be tougher anyway! After the brussels sprouts are cut, chop the red onion into small chunks.
Add all the veggies to a sheet pan and drizzle with olive oil and ½ cup of balsamic vinegar. Sprinkle with 1 tbsp thyme, 2 teaspoon garlic powder, salt and pepper. Set remaining ½ cup of vinegar, ½ tablespoon thyme, and 1 teaspoon garlic powder aside. Place sheet pan in preheated oven and let cook for 15 minutes.
Meanwhile, pound the chicken breasts to tenderize and ensure even thickness, then cut each breast in half.
After 15 minutes, take the pan out to add the chicken and shift the veggies around in the pan so that they cook evenly. Before placing the pan back in the oven, sprinkle the chicken with the remaining thyme, garlic powder, salt, and pepper. Drizzle the extra balsamic vinegar over both the chicken and veggies.
Return the sheet pan to the oven for an extra 20-30 minutes, or until chicken is cooked through and veggies are warm and tender, yet still slightly crisp.
