Preheat the oven to 425°F and line a sheet pan with parchment. In a bowl, whisk the maple syrup, Dijon, vinegar, olive oil, garlic, and paprika until smooth. Reserve 2 Tbsp for finishing.
Place the chicken, Brussels sprouts, squash, and onion on the sheet pan. Drizzle with olive oil, salt, and pepper and toss to coat. Brush the chicken and veggies with half the glaze.
Roast 20 minutes. Remove the pan, brush with the remaining glaze, and return to the oven.
Roast 15-20 minutes more, until the chicken reaches 165°F and the veggies are tender.
Drizzle with the reserved glaze, garnish with parsley, and serve warm.