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1
For tart crust: In a food processor, combine flours and salt.
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2
Pulse to combine.
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3
Add butter and shortening and pulse until mixture resembles moist crumbs; do not overwork dough.
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4
Transfer to a bowl and sprinkle with 4 tablespoons ice water.
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5
Shape into a ball, adding ice water 1 tablespoon at a time (as many as 6 more may be needed) until dough is just past crumbly and holds together.
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6
For two tarts, divide dough in half and wrap each in plastic wrap, and press each with palm of your hand into disks.
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7
For one tart, wrap in plastic and shape into one large disk.
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8
Refrigerate at least 30 minutes before rolling out.
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9
Preheat oven to 400 degrees.
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10
Roll out dough into two 9-inch disks or one 10-by-16-inch rectangle 1/8-inch thick.
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11
Transfer to a baking sheet and crimp edges 1/2-inch high.
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12
Refrigerate for 30 minutes.
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13
Prick all over with fork.
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14
Place parchment paper or foil on top and weigh down with pie weights or dried beans.
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15
Bake until lightly browned, 10 to 15 minutes.
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16
Remove paper or foil and weights from dough, and continue to bake until golden brown, about 15 minutes.
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17
Remove from oven and set aside to cool; do not turn off oven.
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18
For topping: Spread mustard thinly over bottom of cooled shell.
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19
Scatter evenly with cheese.
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20
Arrange tomatoes in even, slightly overlapping rows.
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21
Sprinkle with herbes de Provence, and season to taste with pepper.
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22
Bake until tomatoes begin to shrivel and cheese melts, 10 to 12 minutes.
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23
Sprinkle with salt, and serve hot or at room temperature.
