Preheat the oven to 400°F (200°C) and line a large sheet pan with parchment paper or lightly grease it with oil.
In a small bowl, whisk together Dijon mustard, honey, garlic, olive oil, Italian seasoning, paprika, salt, and black pepper until smooth and well combined.
Place the chicken in a large bowl and pour about half of the honey Dijon sauce over it. Toss until the chicken is fully coated and let it sit while preparing the vegetables.
Add the potatoes, Brussels sprouts, carrots, and red onion to the sheet pan. Drizzle with olive oil and season lightly with salt and pepper, then toss everything together until evenly coated.
Arrange the vegetables in an even layer around the pan, leaving space in the center for the chicken.
Place the coated chicken pieces onto the sheet pan and brush the remaining honey Dijon sauce over the top for extra flavor.
Roast for 30–35 minutes, stirring the vegetables halfway through cooking so they roast evenly and develop golden edges.
Continue cooking until the chicken is fully cooked and the vegetables are tender and caramelized.
Remove from the oven and let the chicken rest for a few minutes before serving.
Garnish with fresh parsley and serve warm directly from the sheet pan.