01 - Set oven temperature to 400°F (205°C) and allow to fully preheat.
02 - Combine carrots, parsnips, potatoes, and red onion in a large bowl. Drizzle with olive oil, sprinkle with rosemary, salt, and pepper. Mix well, then spread evenly in a large baking dish.
03 - In a separate bowl, whisk together Dijon mustard, heavy cream, olive oil, honey, garlic powder, onion powder, thyme, paprika, salt, and pepper until smooth and fully blended.
04 - Pat chicken thighs dry with paper towels. Arrange thighs skin side up over the vegetables in the baking dish. Brush liberally with the prepared Dijon cream sauce, ensuring the thighs are well coated.
05 - Place the baking dish on the middle rack. Bake uncovered for 40–45 minutes or until chicken is cooked through, golden, and juices run clear when pierced. Vegetables should be tender.
06 - Switch oven to broil for 2–3 minutes, watching closely, to further brown chicken skin if desired.
07 - Allow chicken to rest for 5 minutes before serving. Plate with roasted vegetables and spoon over pan sauce as desired.