- 1 (5- to 6-pound) turkey breast with skin and bones
- Fine sea salt
- Freshly ground black pepper
- 1 medium onion, sliced
- 1 cup water
- 3 tablespoons agave nectar
- 1/3 cup sherry vinegar
- 1 cup dry or medium-dry sherry
- 3 cups brown turkey stock, chicken stock, or reduced-sodium broth
- 1/2 cup water
- 1/3 cup all-purpose flour
- 1 to 3 tablespoons sherry vinegar
- Salt
- Freshly ground black pepper
- Special equipment: A roasting pan fitted with a V-shaped rack; an instant-read thermometer
- Pat turkey breast dry and rub all over with 2 1/2 teaspoons sea salt and 1 teaspoon pepper. If you have the time, chill the breast in a baking dish, covered, overnight (see Cooks' Notes).
- Let turkey stand at room temperature 1 hour.
- Heat oven to 425°F with rack in lower third. Position turkey on rack in pan and roast 30 minutes.
- Reduce oven to 400°F and scatter onion slices in bottom of roasting pan. Add 1 cup water and continue to roast until thermometer inserted in thickest part of breast, close to but not touching bone, registers 160°F, 30 to 40 minutes.
- While turkey is roasting, stir together agave nectar and vinegar in a small nonreactive saucepan and boil, stirring occasionally, until thickened and reduced to 3 tablespoons. Remove from heat, but keep warm.
- Brush glaze on turkey and roast until thermometer registers 165°F, 3 to 5 minutes.
- Transfer turkey breast to a platter and let stand 15 minutes.
- While turkey is standing, remove the rack, scraping off any yummy bits into the pan.
- Straddle pan over 2 burners and add sherry, then deglaze pan by boiling sherry, stirring and scraping up browned bits, for 1 minute.
- Strain liquid through a sieve into a heavy medium saucepan. Add stock and bring to a boil. Reduce heat and keep at a simmer.
- Meanwhile, whisk 1/2 cup water slowly into flour in a small bowl until flour slurry is smooth, then whisk in 1 tablespoon vinegar. Add flour slurry to simmering stock, whisking, and bring gravy to a boil, whisking. Reduce heat and simmer, whisking, 3 minutes.
- Season gravy with salt and pepper and additional vinegar, if desired.
- Slice turkey breast and serve with gravy.