Preheat the oven to 400°F (200°C).
In a large bowl, combine the Brussels sprouts, sweet potatoes, and red onion. Drizzle with olive oil and sprinkle with garlic powder, smoked paprika, salt, and pepper. Toss until the vegetables are well coated.
Spread the vegetable mixture evenly on a large baking sheet.
Add the sliced chicken apple sausage on top of the vegetables.
Bake in the preheated oven for 25-30 minutes, or until the vegetables are tender and the sausage is heated through, stirring halfway through the cooking time.
Remove from the oven and let it cool for a few minutes. Garnish with fresh parsley if desired before serving.