Add rice, onion, broth, and soup to a 9x13 baking dish and stir until combined
Season with salt and pepper
Place chicken thighs in rice mixture
Sprinkle with thyme and garlic and (optional) add sliced mushroom and baby spinach leaves.
Cover dish with foil and store in fridge OR cover and bake for 1 hour in oven at 350F.
Uncover and bake 30 minutes more, until rice is tender and chicken is cooked through.
Turn oven to broil and broil until chicken is golden, 3 to 5 minutes.
Garnish with parsley before serving.