Preheat the oven to 375°F and grease a 9x13 inch casserole dish, then finely chop the onion and celery and mince the garlic so everything is ready to cook.
In a bowl, mix flour, parsley, salt, seasoning salt, poultry seasoning, pepper, thyme, and cayenne, then coat each piece of chicken thoroughly in the mixture, saving any leftover flour.
Heat a skillet over medium-high heat with oil, add the chicken in batches without crowding, and sear until browned on both sides before removing it from the pan.
Lower the heat and melt butter in the same pan, then cook the onion and celery until soft before adding garlic and cooking until fragrant.
Sprinkle in the reserved seasoned flour and stir well, then add the broth concentrate and gradually whisk in milk followed by water until a smooth sauce forms.
Rinse the rice until the water runs clear, add it to the prepared casserole dish with water, then stir in the white sauce until evenly combined.
Place the seared chicken on top of the rice mixture, cover the dish tightly with foil, and bake until the rice begins to cook through.
Remove the foil and continue baking, increasing the heat at the end to brown the chicken and allow the casserole to become bubbly and fully cooked.
Let the casserole rest before serving so the sauce thickens and the rice finishes absorbing the liquid, then garnish with parsley.