1Preheat your oven to 350°F. Grease a 9x13-inch baking dish with cooking spray and set aside.
2Melt 2 tablespoons butter in a large skillet over medium-high heat until foaming. Add cubed chicken pieces in a single layer, working in batches if necessary to avoid overcrowding. Season with ½ teaspoon salt and ½ teaspoon black pepper. Sauté until golden brown on all sides and cooked through to 165°F internal temperature, about 7-8 minutes total. Transfer chicken to a large mixing bowl and set aside.
3Return the same skillet to medium heat and add remaining 2 tablespoons butter. Once melted and sizzling, add diced onion and sliced mushrooms. Sauté until onion becomes translucent and mushrooms release their moisture and turn golden, about 6-7 minutes. Add minced garlic and cook until fragrant, stirring constantly, about 45 seconds. Remove from heat.
4Temper the sour cream by whisking it together with ½ cup of warm chicken broth in a small bowl until smooth. This prevents curdling when combined with hot ingredients.
5Combine cream of mushroom soup, tempered sour cream mixture, remaining 3½ cups chicken broth, dried thyme, garlic powder, and remaining ¼ teaspoon black pepper in a large bowl. Whisk until completely smooth and well incorporated.
6Add uncooked wild rice blend, sautéed mushroom mixture, and cooked chicken to the creamy soup mixture. Fold everything together until evenly distributed. Pour entire mixture into prepared baking dish, spreading evenly. Cover tightly with aluminum foil, ensuring edges are sealed to trap steam.
7Bake covered for 50 minutes, then carefully remove from oven and stir gently from bottom to top to redistribute rice and prevent sticking. Re-cover with foil and continue baking for an additional 20 minutes until rice is tender and has absorbed most of the liquid.
8Remove foil and sprinkle shredded cheddar cheese evenly over the top. Return to oven uncovered and bake until cheese melts and turns golden in spots, about 8-10 minutes.
9Let casserole rest for 10 minutes before serving. This allows the sauce to thicken and makes serving much easier.