Preheat oven to 425°F and line a large, rimmed baking sheet with parchment paper or a Silpat® baking mat. Set aside.
Add peppers, Portobello mushrooms, onion, and Brussels sprouts to a large mixing bowl. Add garlic powder, Herbs de Provence, olive oil, and balsamic vinegar and gently toss to combine. Season generously with salt and black pepper, to taste.
Arrange seasoned vegetables on prepared baking sheet in a single layer, being careful not to overcrowd. Use two large baking sheets, if necessary.
Place sheet pan in preheated oven and roast for 40-45 minutes, stirring halfway through, or until the vegetables are tender and develop a nice caramelized color. Remove from oven and serve immediately. Enjoy!