Preheat oven to 400°F and line an extra-large sheet pan (or two smaller sheet pans) with parchment paper. Lightly coat with cooking spray.
In a small bowl, whisk together the olive oil, mustard, vinegar, maple syrup, garlic powder, Italian seasoning, sea salt and pepper.
In a large bowl, combine the apples, sweet potatoes, red onion and Brussels sprouts. Mix in most (about ¾) of the dressing and lightly toss with your hands until the veggies and apples are evenly coated.
Spread the veggies and apples in an even layer on the sheet pan, placing the Brussels sprouts cut side down to properly brown. Bake for 15 minutes.
Add sliced sausage and the remaining amount of Dijon dressing to the now-empty bowl. Toss to coat evenly.
Remove the pan from the oven. Stir in the sausage, ensuring the pan isn’t overcrowded and nothing is touching. Rotate pan and place back in oven. If using two sheet pans, rotate which one is on top.
Bake for an additional 15-20 minutes, until sausage is slightly browned and veggies are tender.
Remove from the oven and let cool for a moment. If desired, add grated Parmesan cheese and fresh parsley. Enjoy!