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1
Bring a large saucepan of salted water to a boil.
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2
Add the pearl onions and blanch for about 1 minute, or until the skins start to loosen.
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3
Drain into a colander and run under cold water until cool to the touch.
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4
Heat the oven to 400 degrees F and arrange a rack in the upper third.
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5
Trim the ends from the onions, cut in half, and peel.
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6
Heat the butter in a Dutch oven or a large, heavy-bottomed pot with a tightfitting lid over medium heat.
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7
When it foams, add onions, leeks, and garlic, season with salt and freshly ground pepper, and cook until leeks are soft.
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8
Add carrots, celery, and thyme and cook until the vegetables are just getting tender.
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9
Add flour, stirring until it is well incorporated, about 1 minute.
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10
Deglaze the pan with the white wine and cook 1 minute more.
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11
Add broth and turkey.
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12
Bring mixture to a simmer, cover, reduce heat to low, and cook, stirring frequently, until vegetables are tender and sauce has thickened slightly, about 15 minutes.
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13
Season with salt and pepper if necessary.
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14
Transfer mixture to a 13-by-9-inch baking dish and spread mashed potatoes over the top.
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15
Bake until the filling is hot and the top is browned, about 35 to 45 minutes.
