Preheat your oven to 400°F (200°C).
Line a large baking sheet with parchment paper or foil for easy cleanup.
Prepare the vegetables. In a large bowl, combine the cubed sweet potatoes, bell peppers, and red onion. Drizzle with olive oil and toss to coat.
Season the vegetables. Sprinkle the Cajun seasoning, garlic powder, smoked paprika, and black pepper over the vegetables. Toss again until evenly coated.
Add the sausage. Add the sliced sausage to the bowl and gently toss everything together.
Make sure the sausage gets some seasoning too.
Spread on the pan. Arrange the mixture in a single layer on the prepared baking sheet. Do not overcrowd the pan. If needed, use two pans.
Roast for 25 to 30 minutes. Halfway through, stir the mixture with a spatula.
Roast until the sweet potatoes are tender and easily pierced with a fork, and the sausage is slightly browned at the edges.
Garnish and serve. Sprinkle with fresh parsley or green onions if desired. Serve hot straight from the pan.