Ingredients

• 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

Instructions

1. 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).
2. Let turkey stand at room temperature 1 hour.
3. Heat oven to 425°F with rack in lower third. Position turkey on rack in pan and roast 30 minutes.
4. 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.
5. For glaze: 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.
6. 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.
7. Brush glaze on turkey and roast until thermometer registers 165°F, 3 to 5 minutes.
8. Transfer turkey breast to a platter and let stand 15 minutes.
9. For gravy: 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.
10. While turkey is standing, remove the rack, scraping off any yummy bits into the pan.
11. Straddle pan over 2 burners and add sherry, then deglaze pan by boiling sherry, stirring and scraping up browned bits, for 1 minute.
12. Strain liquid through a sieve into a heavy medium saucepan. Add stock and bring to a boil. Reduce heat and keep at a simmer.
13. 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.
14. Season gravy with salt and pepper and additional vinegar, if desired.
15. Slice turkey breast and serve with gravy.