Preheat oven to 425°F. Butterfly the chicken breasts so you have 4 thin pieces. Season with salt and pepper.
In a small bowl, whisk together olive oil, balsamic vinegar, maple syrup, dijon mustard, Italian seasoning, garlic powder, salt, and pepper.
Place chicken and cubed butternut squash in a large mixing bowl. Pour in half the sauce and toss to coat evenly.
Spread chicken and squash onto a sheet pan in an even layer. Bake for 10 minutes.
While chicken cooks, toss brussels sprouts and red onion with the remaining sauce. After 10 minutes, add them to the sheet pan, keeping everything evenly spaced.
Bake another 15 minutes, or until chicken reaches 165°F internally and veggies are fork tender.
Remove from oven and serve immediately, or store for meal prep.