Preheat your oven to 375°F and place a rack mid-level. Prepare a 9x13 baking dish by spraying it with nonstick spray or lining it with foil.
Cook the chicken stuffing mixes according to the directions on the box, but swap in 4 tablespoons of butter and 2 1/2 cups water. Let the stuffing cool for around 10 minutes.
Add the frozen cauliflower-vegetable mix, frozen corn niblets, kosher salt, and black pepper into the stuffing and gently fold to combine while the vegetables are still frozen.
Melt the remaining 4 tablespoons of butter in a small bowl. Stir the herbes de Provence and mustard powder into the melted butter.
Place the chicken thighs in a large bowl and toss them with the butter-herb mixture until they are thoroughly coated.
Transfer the vegetable-stuffed mixture into the prepared baking pan and spread it out evenly.
Lay the chicken thighs on top of the stuffing, skin-side up, arranging them evenly.
Bake uncovered for 45 to 60 minutes, turning the pan halfway for even browning, until the chicken reaches 175°F and the stuffing hits 185°F.
If you want crispier chicken skin, switch your oven to broil and place the casserole under the broiler for 2 to 4 minutes. Watch closely to prevent burning.
Serve the casserole hot, and enjoy your comforting, flavorful dinner.