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Preheat the oven to 425°F. Have on hand a 9" (deep dish or regular) pie pan.
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To make the crust: In a medium-sized mixing bowl, whisk together the flour, pecan meal, sugar, salt, and dough improver.
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Cut in the cold butter and vegetable shortening, then toss in enough water for the dough to become somewhat (but not fully) cohesive.
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Sprinkle the dough into the pie pan, and press the crumbs up the sides of the pan.
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To make the filling: Combine all of the filling ingredients in a medium-sized mixing bowl, and spoon them into the prepared crust.
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To make the streusel topping: Combine the brown sugar, flour, oats, and nuts, then mix in the soft butter until the mixture is crumbly.
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Sprinkle the topping over the filling.
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Bake the pie at 425°F for 15 minutes, then reduce the oven heat to 350°F and bake for an additional 35 to 40 minutes, tenting the pie with aluminum foil after 30 minutes if it appears to be browning too quickly.
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Remove it from the oven, and let it cool on a rack before slicing.
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Store the pie, covered, at room temperature for up to 3 days or in the freezer for a month.