Preheat the oven to 425 degrees. Prepare a large, rimmed baking sheet with aluminum foil spritzed with nonstick cooking spray.
Place brussel sprouts, apples, and onion on the baking sheet. Drizzle with olive oil and season with salt and black pepper. Toss well. Spread into an even layer.
Bake for 15 to 20 minutes or until the vegetables reached your desired tenderness.