1. Preheat your oven to 350°F (175°C).
2. In a large pot, cook the diced chicken tenderloin over medium-high heat until browned. Season with Italian seasoning, salt, and pepper. Transfer the chicken to a separate bowl and set aside.
3. Using the same pot, melt 2 tablespoons of butter. Add the diced onions and cook until they are softened, about 5 minutes. Stir in the minced garlic and cook for an additional minute.
4. Pour in the chicken broth and add the remaining butter. Stir in the rice, bringing the mixture to a boil. Once boiling, reduce the heat to low, cover, and let it simmer for 10 minutes.
5. Layer the sliced mushrooms on top of the rice without stirring, cover again, and allow it to cook for another 5 to 10 minutes until the rice is tender.
6. Remove the pot from heat and let it stand, covered, for 10 minutes.
7. Add the cooked chicken, cream of mushroom soup, milk, sour cream, and half of the cheddar cheese to the rice mixture. Stir until well combined.
8. Grease a 9×13 inch casserole dish and transfer the mixture into it. Top with the remaining cheese.
9. Cover the casserole dish and bake in the preheated oven for 20 minutes, or until the cheese is melted and bubbly.
10. Serve and enjoy your delightful Chicken and Rice Bake!