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FILLING:
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Melt butter in a large pot. Add brown and white sugar and water and stir to combine.
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Add flour, cornstarch, cinnamon, nutmeg, lemon zest and vanilla and mix until incorporated.
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Add apple slices and mix carefully until apples are coated with the sauce. Pour into the pie crust.
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TOPPING:
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Put almonds, rolled oats, brown sugar, flour and cinnamon in a food processor. Process until you get fine crumbs. Add butter and pulse a few times. Sprinkle evenly over pie filling. (To get larger crumbs, squeeze topping with your fingers.)
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Bake for 45 minutes in preheated 350 F (175 C) oven.