Preheat oven to 425 degrees F. On a large, rimmed baking sheet, combine sausage, brussels sprouts, sweet potato, apple, and onion. Drizzle avocado oil and toss to coat. Sprinkle with salt and pepper.
Roast for 20-25 minutes until sweet potato is tender and brussels sprouts are crispy on the outside and cooked through. Remove from heat and serve warm.