Preheat the oven to 350°F (175°C).
Heat the olive oil in a large skillet over medium heat.
Add the chopped onion and cook for 3-4 minutes until softened.
Add the minced garlic and sliced mushrooms to the skillet and cook for another 5 minutes until mushrooms are tender.
Add the bite-sized chicken pieces to the skillet and cook until they are no longer pink, about 6-7 minutes.
In a large bowl, combine the cream of mushroom soup, sour cream, chicken broth, dried thyme, salt, and black pepper. Mix well.
Add the cooked chicken mixture, frozen peas, and cooked rice to the bowl and stir until everything is evenly coated with the creamy sauce.
Transfer the mixture to a greased 9×13 inch casserole dish.
Sprinkle the shredded cheddar cheese evenly over the top of the casserole.
Bake in the preheated oven for 25 minutes, or until the casserole is bubbly and the cheese is melted and golden.
Remove from the oven and let it rest for 5 minutes before serving.