Preheat the oven to 400 degrees. Cut the sweet potatoes, onion and apples into bite-sized chunks. Cut the sausage into one-inch coins.
Set the chopped apples aside. Add the potatoes, onion, sausage and garlic to a 9x13 pan. Drizzle with olive oil and maple syrup, and then sprinkle with the rosemary, salt and pepper. Toss well so that everything is evenly coated.
Bake uncovered for 25 minutes. Add the apples, giving everything good stir. Bake an additional 25-35 minutes, or until the potatoes are tender and the sausage is cooked through. If you want everything browned on the top a little more, turn the oven to the broil setting, and broil for about 1-2 minutes.