Preheat oven to 400ºF. Lightly grease a sheet pan - I usually just take a stick of butter and rub all over. Spread the sprouts and sweet potatoes on the sheet pan. Turn the brussels sprouts cut side down so they sear.
Drizzle olive oil over everything, sprinkle with seasoning, then add salt and pepper. Roast for 15 minutes.
Meanwhile, make the sauce: Combine maple syrup, honey and dijon mustard in a bowl or in a jar with a lid and shake well.
Remove the sheet pan from the oven, and add sliced sausage, garlic, and red onion. Drizzle half the sauce over everything and toss with the veggies. Make sure everything is spread out evenly in one layer. Put the sheet pan back into the oven and continue to roast until the veggies are tender.
Remove and drizzle the remaining sauce over the sausage and veggies. Toss to coat. Sprinkle the sausage and veggies with parsley and parmesan. Plate and serve!