This has been one of my favorite easy chicken casseroles for over 40 years. I love that it has the meat and side in one dish. Add a salad and dinner is ready. My Betty Crocker cookbook got alot of use when I was a newlywed back then. She used chicken breasts in her recipe. I found they dry out with that long of a cooking time, so I prefer chicken thighs. If using breasts, I suggest maybe using minute rice instead of long grain. Reduce the cooking time to maybe 30 minutes and then 12-15 minutes after removing the foil. I add the pepper, garlic, & parsley listed, but you can omit if you choose.
Preheat oven to 350 degrees. spray a 9"x13" baking pan with Pam. Blend together soup & milk, reserving 1/2 cup for later use. Add rice, mushrooms, including the liquid, seasonings, and 1/2 of the dry soup mix to mushroom soup mixture. Stir to combine. You can do this in the baking pan(I do) or a bowl.
Arrange the chicken pieces on top of rice mixture. Pour remaining soup & milk mixture over each piece of chicken. Sprinkle remaining dry onion soup mix over chicken pieces. Cover with foil. Bake for 1 hour. Uncover and bake an additional 15 minutes. Add a salad or vegetable for a complete easy meal. Enjoy!