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1
Preheat the oven to 350F.
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2
Use a mandoline to slice the turnips and parsnips into 1/16-inch-thick rounds, and put them into two separate bowls.
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3
Pour 1/2 cup cream onto the bottom of a 9-by-9-inch (or equivalent) gratin dish.
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4
Place one layer of turnips on the bottom of the dish.
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5
(The turnips should overlap by about half.)
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6
Season with 1/4 teaspoon salt and a pinch of pepper.
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7
Scatter a third of the prunes on top.
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8
Arrange a layer of parsnips over the turnips and prunes.
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9
Press the parsnips down with your fingers, letting the cream soak up through the layers.
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10
This will ensure that the cream is evenly distributed and coats the vegetables well.
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11
Drizzle with 1/2 cup cream and season with 1/4 teaspoon salt, a healthy pinch of pepper, and 1 teaspoon thyme.
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12
Arrange another layer of turnips and drizzle another 1/4 cup cream over them.
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13
Season with 1/4 teaspoon salt, a pinch of pepper, and 1 teaspoon thyme.
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14
Scatter a third of the prunes on top and continue with another layer of parsnips.
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15
Drizzle on 1/2 cup cream and season with 1/4 teaspoon salt and a pinch of pepper.
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16
Press the vegetables down with your fingers, allowing the cream to come up through the layers and coat the vegetables evenly.
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17
Finish the gratin with one more layer, this time of both parsnip and turnip slices, arranging this layer nicely, since it will be the top of your gratin.
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18
Scatter the remaining prunes over the top.
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19
Drizzle with 1/4 to 1/2 cup cream and season with 1/4 teaspoon salt, freshly ground black pepper, and the remaining teaspoon thyme.
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20
Press the gratin down with your fingers again.
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21
The cream should cover the potatoes but not be too soupy.
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22
Add more cream if the gratin seems dry.
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23
Cover tightly with aluminum foil.
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24
Bake about 1 1/2 hours, until the vegetables are tender when pierced.
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25
Remove from the oven and carefully uncover.
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26
Turn the oven to 425F and return the gratin to the oven.
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27
Cook another 15 to 20 minutes, until the top is nice and golden brown (as in gratineed).
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28
For a nice-looking gratin, look for parsnips and turnips that are about the same size.
