Preheat the oven to 425°F (220°C). Line a large rimmed sheet pan with parchment paper or lightly oil the pan.
Whisk the olive oil, Dijon mustard, maple syrup, apple cider vinegar, garlic powder, smoked paprika, thyme, salt, and black pepper. Reserve about one third for the sausage.
Toss the butternut squash, Brussels sprouts, and red onion with the larger portion of glaze. Spread them in a loose single layer, placing the sprouts cut-side down when possible.
Roast the vegetables for 15 minutes to give the dense squash a head start.
Coat the sliced fully cooked sausage with the reserved glaze. Stir the vegetables and arrange the sausage throughout the pan.
Roast for another 15 to 20 minutes, until the squash is fork-tender, the Brussels sprouts are browned, and the sausage is hot and caramelized.
Taste and adjust the seasoning. Sprinkle with fresh parsley and serve hot.