Preheat the oven to 350°F (175°C).
Heat olive oil in a large skillet over medium heat.
Add the diced onion and cook for 3-4 minutes until softened.
Add the minced garlic and sliced mushrooms and cook for another 3-4 minutes until mushrooms are tender.
Add the chicken pieces to the skillet and cook until they are no longer pink in the center, about 7-8 minutes.
Stir in the frozen peas, carrots, and corn, cooking for 2-3 minutes until vegetables are slightly thawed.
In a large mixing bowl, combine the cooked chicken and vegetable mixture with the cooked rice.
Add the condensed cream of chicken soup, chicken broth, sour cream, dried thyme, dried parsley, salt, and black pepper to the bowl. Mix everything thoroughly until well combined.
Transfer the mixture into a greased 9×13 inch casserole dish and spread evenly.
Sprinkle the shredded cheddar cheese evenly over the top of the casserole.
Bake in the preheated oven for 35-40 minutes, or until the casserole is bubbly and the cheese is melted and golden brown.
Remove from the oven and let it rest for 5 minutes before serving.