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1
In a medium bowl, whisk together broth, wine, tomato paste, and flour until evenly combined and smooth.
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2
Stir in dried mushrooms and set aside to reconstitute, at least 30 minutes.
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3
Strain mushrooms before using, reserving liquid.
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4
Place potatoes in a large pot and cover with heavily salted water by 2 inches.
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5
Bring potatoes to a boil and cook until fork tender, about 20 to 30 minutes.
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6
Heat the oven on low broil and place a rack in the upper third.
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7
Meanwhile, melt 1 tablespoon of the butter in a 3 to 4-quart Dutch oven (or oven-ready saucepan) over medium-high heat.
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8
When it foams, add half the mushrooms and cook, stirring rarely, until mushrooms are browned, about 5 minutes.
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9
Remove mushrooms from pan, season well with salt and freshly ground black pepper, and set aside.
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10
Repeat to cook off remaining mushrooms.
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11
Return pan to stove over medium heat and add 1 tablespoon of the butter, onion, celery, and garlic, and cook until softened and golden, about 2 minutes.
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12
Add celery root, carrot, parsnip, and herbs, and season well with salt and freshly ground black pepper.
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13
Cook until browned and softened, about 6 minutes.
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14
Add wine mixture to pan and deglaze by stirring and scraping up any browned bits.
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15
Let cook until simmering and slightly thickened, about 3 minutes.
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16
Stir in reserved mushrooms and any juices that have accumulated and simmer until slightly thickened, about 8 minutes.
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17
Remove from heat and reserve in pan.
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18
When potatoes are ready, drain well.
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19
Return to pan and mash until uniformly smooth.
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20
Fold in remaining 2 tablespoons butter and milk, and season well with salt and freshly ground black pepper.
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21
If necessary, keep warm over low heat.
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22
Dot potatoes over vegetable mixture and spread to edges of pan to cover completely.
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23
Rough up the surface of the potatoes so there are bits that will get browned and crunchy.
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24
Broil until top is golden, about 15 to 20 minutes.
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25
Serve.
