1 whole boneless turkey breast (6 to 7 pounds)
2 teaspoons onion salt
2 teaspoons chili powder
1 teaspoon dried oregano
1 teaspoon ground cumin
1/2 teaspoon garlic powder
1/4 teaspoon ground allspice
1/4 teaspoon cayenne pepper
2 tablespoons all-purpose flour
1 can (14.5 ounces) chicken broth
Preheat oven to 350 degrees F.
Coat large roasting pan with nonstick vegetable-oil cooking spray. Rub turkey breast on all sides with dry rub. Roll the turkey breast up and tie with butcher's twine. Place in roasting pan.
Roast turkey in the oven for 1 1/2 to 2 1/2 hours or until the internal temperature registers 165 degrees F on instant-read thermometer. Remove turkey to a warm place; let stand 10 minutes.
Gravy: Pour off all but 2 tablespoons drippings from roasting pan. Sprinkle flour over bottom of roasting pan; cook over medium-high heat, scraping up any browned bits from bottom of pan, for about 1 minute. Gradually whisk in chicken broth until smooth; cook, stirring occasionally, until thickened, about 2 minutes. Strain gravy through sieve into gravy boat; keep warm.
Cut off each 1/2 of the turkey breast from the bone. Wrap one 1/2 and refrigerate for up to 3 days or freeze for up to 4 months, for later use. Slice other 1/2 of turkey breast and serve with the gravy, and mashed potatoes, if desired.