Line a large, rimmed baking sheet with a silicone baking mat or parchment paper, and then set aside. Preheat oven to 425°F.
To a large mixing bowl, add Portobello mushrooms, mini peppers, onion, and Brussels sprouts. Add Herbs de Provence, garlic powder, olive oil, and balsamic vinegar. Gently toss to combine. Generously season with salt and black pepper to taste.
Onto the prepared baking sheet, arrange seasoned vegetables in a single layer. Be careful not to overcrowd them. (If necessary, use two large baking sheets to prevent overcrowding.)
Place sheet pan in oven. Roast for 40-45 minutes, stirring halfway through. The vegetables should be roasted until they’re tender and have developed a caramelized color.
Remove from oven and serve immediately.
If necessary, you can use Italian seasoning instead of Herbs de Provence. The flavor will be slightly altered, but still delicious.