Set your oven to 375°F and position a rack in the center. Prepare a 9- x 13-inch baking dish by spraying it with nonstick spray or lining it with foil.
Follow the instructions on the stuffing mix boxes, but use 4 Tbsp. of butter and 2 1/2 cups of water. Let the prepared stuffing cool for 10 minutes.
Stir the frozen cauliflower rice, frozen corn niblets, kosher salt, and black pepper into the cooled stuffing mix until well combined.
Melt the remaining 4 Tbsp. of butter in a small bowl. In a large bowl, combine the melted butter with herbes de Provence and ground mustard powder.
Coat each chicken thigh thoroughly with the butter and herb mixture.
Spread the stuffing and vegetable mixture evenly in the prepared baking dish.
Arrange the chicken thighs skin-side up over the stuffing layer.
Bake uncovered for 45 minutes to 1 hour, checking that the chicken reaches an internal temperature of 175°F and the stuffing is at 185°F. Rotate the pan halfway through the baking time for even cooking.
If the chicken skin needs extra crisping, switch your oven to broil and place the casserole under the broiler for 2 to 4 minutes. Watch carefully to avoid burning.
Serve the casserole hot and enjoy a comforting meal that's perfect for any day of the week.