Preheat your oven to 350°F and lightly grease a 9x13 inch baking dish. Pat the chicken pieces dry with paper towels to ensure even browning and better texture throughout cooking.
Heat butter in a large skillet over medium-high heat, then sear chicken pieces for 3-4 minutes per side until golden brown. Remove chicken to a plate and set aside, keeping all those delicious browned bits in the pan.
In the same skillet, sauté diced onion and garlic for 2 minutes until fragrant and softened. Add sliced mushrooms and cook for another 3 minutes, stirring occasionally until they release their moisture.
Add uncooked rice to the golden tender chicken rice casserole base, stirring to coat each grain with the buttery mixture. Pour in chicken broth, dried thyme, salt, and pepper, then stir everything to combine evenly.
Transfer the rice mixture to your prepared baking dish, arranging seared chicken pieces on top in a single layer. Cover tightly with aluminum foil and bake for 35 minutes until rice is tender and liquid is mostly absorbed.
Remove foil, then stir in frozen peas and diced carrots throughout the rice. Drizzle heavy cream over everything and sprinkle shredded cheddar cheese evenly across the top.
Return the casserole to the oven uncovered and bake for 5 more minutes until cheese melts and creates a golden, bubbly layer. Garnish with fresh parsley before serving for color and brightness.