Combine 8 cups water, salted as you would for pasta, and Grano in a pot. Bring to a boil, then reduce heat to medium. Simmer, stirring frequently, until Grano is tender but still chewy, about 30-35 minutes. Drain well.
Heat oven to 350°F. Blend soup and milk; reserve 1/2 cup of mixture. Stir together remaining soup mixture, drained Grano, mushrooms with liquid and half of the onion soup mix. Pour into a greased baking dish (11Óx 17Ó). Arrange chicken breast on the Grano mixture. Pour reserved soup mixture over chicken and sprinkle with remaining onion soup mix. Cover and bake for 1 hour. Uncover and bake for 15 minutes longer.