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1
Preheat oven to 425F.
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2
To prepare crust:
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3
Put walnuts in a food processor, pulse until finely ground.
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4
Mix in a large bowl with whole-wheat flour, all-purpose flour, 2 tablespoons thyme and/or rosemary, 3/4 teaspoon each salt and pepper.
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5
Make a well in the center and add 1/2 cup oil and water.
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6
Gradually add the wet ingredients into the dry ingredients to form a soft dough (it will seem wetter than other types of pastry dough).
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7
Knead in the bowl just until the dough comes together.
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8
Pat it into a disk, wrap in plastic and refrigerate for at least 15 minutes or up to 3 days.
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9
To prepare filling:
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10
Add sweet potatoes, 1 tablespoon oil, 1/2 teaspoon salt and 1/4 teaspoon pepper in a large bowl and stir well.
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11
Spread on three-fourths of a large rimmed baking sheet.
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12
Toss onion in the bowl with 1 teaspoon oil.
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Spread evenly on the remaining one-fourth of the baking sheet.
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14
Roast for 10 minutes.
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15
Remove from the oven.
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16
Reduce temperature to 375.
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17
Line a work surface with parchment paper or a nonstick baking mat, lightly dust with flour and dust the top of the dough with flour.
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18
Roll the dough into a rustic 15-inch circle, adding more flour, if necessary, to prevent sticking.
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19
Transfer the crust to a baking sheet with the parchment or baking mat.
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20
Leaving a 2-inch border, sprinkle cheese evenly over the crust.
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21
Make an overlapping ring of the larger sweet potato slices over the cheese, leaving the 2-inch border.
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22
Spread the onion slices evenly in another ring closer to the center.
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23
Using the rest of the sweet potato slices, make an overlapping circle in the center of the crust (the pattern will look like a bulls-eye).
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24
Pick up the edges of the crust using a spatula and fold over the filling, making pleats in the dough as necessary (its okay if the dough cracks a little as you fold it); the filling will not be completely covered.
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25
Brush the crust with the egg-white wash.
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26
Drizzle the vegetables with the remaining 1 teaspoon oil and sprinkle with 1 teaspoon thyme and/or rosemary.
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27
Bake the tart until lightly browned on the edges, about 50 minutes.
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28
Cool for about 8 minutes.
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29
Slice and serve warm.
