Preheat the oven to 425 degrees. Add parchment paper to a large, rimmed backing sheet. Set aside.
Add the mushrooms, peppers, Brussels sprouts, and onion to a large mixing bowl. Add the olive oil, garlic powder, Herbs de Provence, and balsamic vinegar. Toss to combine.
Season with salt and pepper.
Put the veggies onto the baking sheet in a single layer. Don't overlap or overcrowd. You might need to use two baking sheets.
Put the pan in the oven and roast for 40-45 minutes. Stir halfway through, and cook until the veggies are tender and start to caramelize.