Preheat the oven to 375°F (190°C).
Heat olive oil in a large skillet over medium heat.
Add diced onion and cook for 3-4 minutes until softened.
Add minced garlic and cook for 1 minute until fragrant.
Add chicken pieces to the skillet and cook for 5-6 minutes until lightly browned but not fully cooked through.
In a large mixing bowl, combine the rinsed rice, chicken broth, frozen peas, sliced mushrooms, sour cream, dried thyme, paprika, salt, and black pepper. Stir well to combine.
Add the cooked chicken, onion, and garlic mixture to the rice mixture and stir to combine evenly.
Transfer the entire mixture into a greased 9×13 inch casserole dish and spread evenly.
Cover the casserole dish tightly with aluminum foil.
Bake in the preheated oven for 35 minutes.
Remove the foil and sprinkle shredded cheddar cheese evenly over the top.
Return the casserole to the oven uncovered and bake for an additional 10 minutes, or until the cheese is melted and bubbly and the rice is tender.
Remove from oven and let the casserole rest for 5 minutes before serving.