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1
Preheat oven to 350u00b0F.
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2
Stir together flour, salt and ground walnuts.
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3
With a mixer, beat butter, sugar, and vanilla until pale and fluffy, about 2-3 minutes. Slowly add walnut mixture, and beat until just combined.
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4
Press the dough into the tart pan with your fingers until it is evenly distributed on the bottom and sides of pan. Chill for 15 minutes in the freezer. Prick with a fork all over.
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5
Place the pan in preheated oven and bake for 20 minutes or until lightly browned.
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6
Remove from oven, and cool completely.
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7
Mix together sugar and mascarpone. Layer the mascarpone mixture onto the cooled crust.
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8
Stir the sugar, 2 tablespoons water, lemon juice, and 2 tablespoons of the cornstarch/water mixture into a saucepan over medium heat. Simmer for 1-2 minutes until thickened and sugar has dissolved.
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9
Add 2 cups of blueberries, then bring to a boil and simmer until juicy and thick - about 4 minutes (if they get too thick then add a little water to thin them out).
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10
Take the pan off the heat and add the remaining 2 cups of berries. Gently fold them in, coating completely.
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11
Spread the berry topping over the cream base and let set for 5-10 minutes. Top with lemon zest and serve.
