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1
Pat turkey breast dry and rub all over with 2 1/2 teaspoons sea salt and 1 teaspoon pepper.
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2
If you have the time, chill the breast in a baking dish, covered, overnight (see Cooks' Notes).
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3
Let turkey stand at room temperature 1 hour.
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4
Heat oven to 425F with rack in lower third.
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5
Position turkey on rack in pan and roast 30 minutes.
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6
Reduce oven to 400F and scatter onion slices in bottom of roasting pan.
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7
Add 1 cup water and continue to roast until thermometer inserted in thickest part of breast, close to but not touching bone, registers 160F, 30 to 40 minutes.
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8
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.
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9
Remove from heat, but keep warm.
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10
Brush glaze on turkey and roast until thermometer registers 165F, 3 to 5 minutes.
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11
Transfer turkey breast to a platter and let stand 15 minutes.
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12
While turkey is standing, remove the rack, scraping off any yummy bits into the pan.
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13
Straddle pan over 2 burners and add sherry, then deglaze pan by boiling sherry, stirring and scraping up browned bits, for 1 minute.
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14
Strain liquid through a sieve into a heavy medium saucepan.
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15
Add stock and bring to a boil.
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16
Reduce heat and keep at a simmer.
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17
Meanwhile, whisk 1/2 cup water slowly into flour in a small bowl until flour slurry is smooth, then whisk in 1 tablespoon vinegar.
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18
Add flour slurry to simmering stock, whisking, and bring gravy to a boil, whisking.
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19
Reduce heat and simmer, whisking, 3 minutes.
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20
Season gravy with salt and pepper and additional vinegar, if desired.
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21
Slice turkey breast and serve with gravy.
