Preheat the oven to 400 degrees.
In a large bowl sift together flour and salt. Remove 1/4 cup flour to a small bowl and add the water, making a paste.
With a pastry cutter, cut the shortening into the flour in the large bowl. Add the paste and mix well.
On a floured surface, roll out the pie crust to fit an 11-inch tart pan with a removable bottom.
Prick the pie crust all over with a fork or use pie weights so that the crust doesn't bubble up while cooking. Bake for about 15 minutes, until lightly golden, removing the pie weights after 13 minutes. Remove from the oven and cool on a wire rack.
In a food processor, puree the basil, olive oil, garlic and a pinch of salt. Set aside. In a small bowl, mix the mascarpone cheese with the lemon juice. Once the pie crust has cooled, spread the cheese on the bottom. Arrange the tomatoes on top of the cheese, sprinkle with salt and pepper and drizzle the basil oil on top. Cut into wedges and serve.
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