Preheat oven to 350 degrees F.
Coat large roasting pan with nonstick vegetable-oil cooking spray. Rub turkey on all sides with olive oil and lime juice.
Dry Rub: Mix onion salt, chili powder, dried oregano, cumin, garlic powder, allspice and cayenne in small bowl. Rub 1 1/2 tablespoons mixture over turkey.
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 and 1/2 teaspoon of remaining dry rub 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.