Preheat the oven to 425°F. Line an extra-large rimmed baking sheet with parchment paper and set aside.
In an extra-large bowl, combine the Brussels sprouts, sweet potatoes, red onion, apples, and sliced chicken sausage.
Drizzle with half of the oil and season with the salt, pepper, and Italian seasoning. Toss everything together until evenly coated.
Spread the mixture evenly across the prepared baking sheet. Roast for 25 minutes.
While the vegetables and sausage are roasting, whisk together the maple syrup, Dijon mustard, and apple cider vinegar in the same bowl. Season with a little salt and pepper.
Remove the sheet pan from the oven. Drizzle the maple-Dijon sauce over the roasted vegetables and sausage. Stir everything together to coat, then spread it back into an even layer.
Return the pan to the oven and roast for another 20 minutes, or until the vegetables are tender and the chicken sausage is browned.
Taste and add additional salt and pepper if desired. Serve warm.