Preheat oven to 350°.
Combine the onion, carrot, and celery in a roasting pan coated with cooking spray. Add 1 tablespoon soy sauce and 2/3 cup broth; stir to coat. Place turkey breast, skin side up, on vegetables. Sprinkle with pepper and salt. Insert meat thermometer into turkey breast, making sure it does not touch bone. Bake at 350° for 1 hour; baste turkey with 2 tablespoons broth every 30 minutes. Combine 2 tablespoons soy sauce, sherry, and molasses in a small bowl. Bake turkey an additional 45 minutes or until thermometer registers 180°, brushing with sherry mixture every 15 minutes. Place turkey on a platter. Cover turkey loosely with foil; let stand 15 minutes. Do not discard drippings.
Combine the remaining chicken broth and flour, stirring with a whisk until well blended to form a slurry. Drain the onion mixture and drippings into a colander over a bowl, reserving both. Place a zip-top plastic bag inside a 2-cup glass measure or bowl. Pour drippings into bag; let stand 10 minutes (fat will rise to the top). Seal bag; carefully snip off 1 bottom corner of bag. Drain the drippings into a medium saucepan, stopping before fat layer reaches opening; discard fat. Add the reserved onion mixture to pan; stir in slurry. Bring to a boil; reduce heat and simmer 5 minutes. Serve turkey with gravy.