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1
Preheat oven to 450F.
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2
Put lamb shanks in a large metal roasting pan, then rub with oil and season with salt and pepper.
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3
Arrange onion wedges around lamb.
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4
Roast lamb in middle of oven 40 minutes.
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5
Turn shanks over, scatter with herb sprigs, and roast 40 minutes more.
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6
Pour wine, broth, and water into roasting pan.
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7
Cover pan tightly with foil and braise lamb until tender, 45 minutes to 1 hour.
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8
Transfer shanks to a plate and remove and discard skins from onions.
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9
Pour cooking liquid (including onions) into a large glass measure (do not clean roasting pan).
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10
Pierce each potato once with a fork and bake on rack in lower third of oven until cooked through, 45 to 50 minutes.
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11
Cool potatoes 10 minutes, then halve lengthwise and scoop out flesh.
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12
Force warm potatoes through ricer into a bowl.
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13
Stir in salt, milk, and pepper to taste.
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14
Cook carrots in boiling salted water until tender, about 10 minutes.
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15
Rinse under cold running water to stop cooking.
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16
Skim fat from cooking liquid (you'll have about 2 1/2 cups broth).
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17
Whisk together 1 cup broth and flour in a large bowl to make a thin paste, then whisk in remaining broth (including onions).
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18
Set roasting pan across 2 burners and pour broth mixture into pan.
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19
Boil over moderate heat, whisking, until thickened, about 5 minutes.
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20
Remove from heat and season with salt and pepper.
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21
Cut lamb meat from bones, then tear meat into bite-size pieces.
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22
Stir meat into gravy.
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23
Reduce oven to 350F.
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24
Spoon lamb-shank mixture into a 12- by 3-inch oval gratin dish or a 2 1/2- to 3-quart shallow baking dish, spreading evenly.
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25
Scatter carrots over lamb, then top with creamed spinach, spreading evenly.
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26
Top spinach with mashed potatoes, spreading evenly to edges of dish to cover filling completely.
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27
Make swirl patterns on surface of potatoes with back of spoon, then drizzle with melted butter.
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28
Put baking dish in a foil-lined shallow (1-inch-deep) baking pan to catch drips and bake until top is golden and filling is bubbling, about 1 hour.
