Preheat oven to 425°F (220°C) with a rack in the center position. Line a large sheet pan with aluminum foil or parchment paper, and place an oven-safe wire rack on top if using.
In a medium bowl, whisk together the Dijon mustard, whole grain mustard, honey, 1 tablespoon olive oil, apple cider vinegar, minced garlic, onion powder, smoked paprika, dried thyme, and salt and pepper until smooth and glossy. Reserve 2 tablespoons of the glaze in a separate small bowl for basting later.
Pat the chicken thighs completely dry with paper towels, then season both sides with salt and black pepper. Spoon the honey mustard glaze generously over each piece, pressing some under the skin, and place skin-side up on the prepared pan.
Toss the halved baby potatoes, broccoli florets, sliced bell pepper, and red onion wedges in a bowl with the remaining 1 tablespoon olive oil and salt and pepper. Arrange the vegetables in a single layer around the chicken, placing potatoes cut-side down.
Roast in the preheated oven for 20 minutes undisturbed, allowing the chicken to begin browning and the vegetables to start softening.
Remove the pan from the oven, flip the vegetables with tongs, and brush the reserved (clean) honey mustard glaze over the top of each chicken piece. Return to the oven.
Continue roasting for another 15–20 minutes, until the chicken reaches an internal temperature of 165°F (74°C), the skin is deep golden and caramelized, and the potatoes are fork-tender. Broil for 2–3 minutes at the end if more caramelization is desired.
Remove the pan from the oven and let the chicken rest for 5 minutes. Garnish with fresh parsley or thyme sprigs and serve directly from the pan.