Step 1: Saut the Aromatics: Preheat oven to 375F (190C). In a large oven-safe skillet or Dutch oven, heat olive oil over medium heat. Add chopped onion and cook until softened, about 5 minutes. Stir in minced garlic and cook for another minute until fragrant. Ensure not to burn the garlic, as it will turn bitter.
Step 2: Brown the Chicken: Add the cut chicken pieces to the skillet. Cook until browned on all sides. The chicken does not need to be fully cooked at this stage, as it will finish cooking in the oven.
Step 3: Combine Ingredients: Stir in the uncooked rice, chicken broth, condensed cream of mushroom soup, milk, Parmesan cheese, dried thyme, salt, and pepper. Mix well to ensure the rice is evenly distributed. If using, stir in frozen peas or mixed vegetables at this point.
Step 4: Bake the Casserole: Bring the mixture to a simmer. Cover the skillet or Dutch oven with a lid or aluminum foil. Bake in the preheated oven for 45 minutes, or until the rice is cooked through and the liquid is absorbed. Check the casserole halfway through cooking and add a little more chicken broth if it seems dry.
Step 5: Rest and Serve: Remove from the oven and let it rest for 5-10 minutes before serving. This allows the casserole to thicken slightly. Garnish with fresh parsley, if desired, and serve hot.
For a richer flavor, substitute heavy cream for milk. For a vegetarian option, replace the chicken with mushrooms or other vegetables. Using pre-cooked rice will significantly reduce baking time, but adjust liquid accordingly.