Generously season both sides of the chicken thighs with mustard powder, paprika, onion powder, salt, and pepper.

Prepare the brown rice according to package instructions, either using the microwave or stovetop. Set aside once cooked.

Heat 1 tablespoon of olive oil in a medium skillet over medium heat.

Place the chicken thighs skin side down and brown for 4 minutes until golden. Flip and brown the other side for another 4 minutes. Transfer chicken to a plate to rest.

Add the remaining tablespoon of olive oil to the skillet. Sauté the chopped onion and minced garlic for about 3 minutes until softened and fragrant.

Lightly spray the bottom of a 9 x 13-inch casserole dish with non-stick cooking spray.

Spread the cooked brown rice evenly as the base layer.

Arrange the browned chicken thighs on top of the rice evenly.

In a large bowl, combine the sautéed onion and garlic, cream of mushroom soup, condensed cream of chicken soup, milk, sliced mushrooms, salt, and pepper to taste. Stir well to combine.

Pour the creamy mushroom and chicken soup mixture over the chicken and rice in the casserole dish.

Sprinkle dry parsley over the top.

Cover the dish with foil and bake in the preheated 350°F (175°C) oven for 45 minutes.

Remove the foil and bake uncovered for an additional 20 minutes until the chicken is fully cooked, the sauce has thickened, and the top is lightly browned.