In a large bowl, whisk together the olive oil, Dijon mustard, maple syrup, and apple cider vinegar.
Toss the Brussels sprouts, sweet potatoes, carrots, onion, and sausage slices in the mixture until all the vegetables and sausage are evenly coated. Season with salt and pepper.
Spread the Brussels sprouts, sweet potatoes, carrots, onion, and sausage on a baking sheet in a single layer.
Roast for 30-35 minutes, tossing halfway through, until the vegetables are tender and caramelized, and the sausage is browned.
Optional: Garnish with chopped pecans for extra crunch before serving.