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In a small, heavy saucepan, combine the rhubarb, 1/2 cup of the sugar, and 1 slice cut from the lemon.
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Bring to a boil over high heat, stirring until the sugar melts.
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Reduce the heat to medium and simmer 12 to 13 minutes or until thickened and reduced to about 1/2 cup.
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Let cool.
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In the bowl of a food processor, combine the flour and 3/4 cup of the sugar.
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Add the butter and pulse until the mixture resembles coarse crumbs.
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Measure 1 cup of the mixture into a small bowl and add the walnuts and cinnamon.
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Set aside.
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Add the baking powder, baking soda, salt, sour cream, vanilla and one egg to the remaining flour/butter mixture in the food processor.
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Pulse until a soft, sticky dough is formed.
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With floured fingers, press the dough evenly over the bottom and 2 inches up the sides of a greased 10" springform pan.
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Wipe out the bowl of the food processor and add 1 teaspoons grated lemon rind, the cream cheese, 1/4 cup sugar and the remaining egg.
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Pulse until well blended.
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Spread the cream cheese mixture over the dough in the cake pan.
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Top with the rhubarb mixture.
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Sprinkle the walnut mixture evenly over the top.
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Bake in a 350 degree F oven for about 1 hour or until a knife inserted in the centre of the cake comes out clean.
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Place the pan on a wire rack and let cool for 15 minutes.
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Remove from the pan.
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Serve warm, cut into wedges and refrigerate any leftovers.