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Have the Pie Crust dough ready to use.
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Position rack in middle of oven. Preheat oven to 375°F/190°C. Coat a 15-inch x 10-inch x 1-inch (38 cm x 25 cm x 2.5 cm) rectangular rimmed baking pan with nonstick spray.
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Roll out the pie dough on a lightly floured surface or between two pieces of parchment paper into a rectangular shape and transfer to the prepared pan; it is fine to do this in a few pieces. For slab pies I often press some of it in using floured fingertips into the leftover spaces. Chill crust for 15 minutes. Line the crust with aluminum foil and pie weights and bake for 12 minutes or until the crust is just beginning to set and turns light brown around the edges.
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For the Fruit Filling: While the crust is pre-baking, make the fruit filling. Toss fruit together in a medium sized mixing bowl. Add sugar, cornstarch and lemon juice and use a silicone spatula to combine well. Let sit for 5 to 10 minutes or until fruit juices exude, then fold together well again.
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For the Crumb Topping: While the fruit is resting, make the crumb topping.
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Melt the butter in a medium sized microwave safe mixing bowl (or melt on stovetop and transfer to mixing bowl). Whisk in brown sugar, then whisk in gluten free flour, oats, cinnamon and salt until well combined. Use hands to help form clumps.
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Scrape fruit and any juice into pre-baked bottom crust and spread into even layer. Sprinkle crumb topping evenly over fruit.
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Bake for about 35 to 40 minutes or until edges of crust and crumb topping are golden brown and the fruit is bubbling. Cool on rack for about 10 minutes. Slab pie may be served warm or at room temperature. It is best eaten the day it is made but may be stored overnight at room temperature lightly covered with foil.