Preheat the oven to 425°F (220°C).
Pat the chicken thighs dry with paper towels, then season both sides generously with salt and black pepper.
In a small bowl, whisk together the olive oil, whole grain Dijon mustard, regular mustard, honey, minced garlic, and red pepper flakes until smooth.
Place the seasoned thighs in a large mixing bowl, pour the sauce over them, and toss well to coat each piece evenly.
Arrange the coated thighs skin-side up in a 9×13-inch baking dish, including any remaining sauce; no need to grease the dish.
Bake in the preheated oven for about 35 minutes, or until an instant-read thermometer inserted into the thickest part of a thigh (not touching bone) registers 165°F and the skin is crisp.
Remove the dish from the oven, transfer the thighs to a serving plate, tent with foil, and let rest for 10 minutes before serving.