Preheat oven to 425°F (220°C) and line a large rimmed baking sheet with parchment paper or foil.
Whisk together Dijon mustard, whole-grain mustard, honey, olive oil, apple cider vinegar, garlic, paprika, rosemary, salt, and pepper until smooth. Reserve 2 tablespoons for basting.
Pat chicken thighs completely dry, then coat with half the sauce in a bowl. Let marinate while prepping vegetables.
Toss halved potatoes, green beans, bell peppers, and onion wedges with vegetable oil, salt, and pepper, then drizzle with remaining sauce.
Spread vegetables evenly on the baking sheet and nestle chicken thighs on top, skin-side up.
Roast for 20 minutes, then baste chicken with reserved sauce, flip vegetables, and continue roasting 15-20 minutes until chicken reaches 165°F (74°C).
Let chicken rest for 5 minutes, garnish with fresh parsley, and serve directly from the pan.