Step 1: Preheat Oven and Prepare Baking Dish: Preheat oven to 375F (190C). Lightly grease a 9x13 inch baking dish.
Step 2: Saut Aromatics: Heat olive oil in a large skillet over medium heat. Add chopped onion and cook until softened, about 5 minutes. Add minced garlic and cook for another minute until fragrant.
Step 3: Cook Chicken: Add the chicken pieces to the skillet and cook until lightly browned on all sides. The chicken does not need to be fully cooked at this stage.
Step 4: Combine Ingredients: In the prepared baking dish, combine the uncooked rice, cooked chicken and onion mixture, chicken broth, cream of chicken soup, milk, mixed vegetables, dried thyme, salt, and pepper. Stir well to ensure all ingredients are evenly distributed.
Step 5: Bake Casserole: Cover the baking dish tightly with aluminum foil and bake in the preheated oven for 30 minutes.
Step 6: Add Cheese (Optional) and Finish Baking: Remove the foil and sprinkle shredded cheddar cheese (if using) evenly over the top of the casserole. Return to the oven and bake uncovered for an additional 15 minutes, or until the cheese is melted and bubbly, and the rice is cooked through. The internal temperature of the chicken should reach 165F (74C).
Step 7: Rest and Serve: Remove the casserole from the oven and let it rest for 5-10 minutes before serving. This allows the casserole to set slightly and the flavors to meld together.
Optional tips or substitution notes, if relevant: For a richer flavor, use bone-in, skin-on chicken thighs. Increase cooking time by 15-20 minutes. Brown rice can be substituted for white rice, but increase the chicken broth to 3 cups and baking time to 50-60 minutes. A can of drained diced tomatoes can be added for extra flavor.