On a rimmed sheet tray, add onions, sweet potatoes, bell peppers and sausages. Poke a hole in each sausage. Drizzle with oil, salt, pepper, paprika, garlic powder and rosemary. Gently toss with your hands to coat. Spread in a single layer.
Roast for 20 to 25 minutes, tossing halfway. Remove from oven, and allow to cool slightly. Serve with mustard.