Preheat oven to 425°F. Line a large rimmed baking sheet with parchment paper or Silpat.
Add peppers, mushrooms, onion, and Brussels sprouts to a large mixing bowl.
Add garlic powder, Herbs de Provence, olive oil, and balsamic vinegar. Toss to combine. Season generously with salt and pepper.
Arrange vegetables on prepared baking sheet in a single layer. Don’t overcrowd! Use two sheets if necessary.
Roast 40-45 minutes, stirring halfway through, until vegetables are tender and caramelized.
Serve immediately.