Heat oven to 350 degrees. Mix mushroom soup and milk; reserve 1/2/cup of the mixture. Mix remaining soup mixture, the rice, mushrooms with liquid, and half the onion soup mix.
Pour into ungreased baking dish, 11 1/2 x 7 1/2 x 1 1/2 inches. Place chicken breasts on top. Pour reserved soup mixture over chicken breasts; sprinkle with remaining onion soup mix. Cover with aluminum foil; bake 1 hour. Uncover; bake 15 minutes longer.