Preheat your oven to 375°F (190°C). Grease a 9x13-inch baking dish or line it with parchment paper.
In a large bowl, gently mix together the cherry plums, cherries, granulated sugar, cornstarch, lemon juice, and vanilla extract until the fruit is evenly coated. Set aside.
Roll out one of the pie crusts and place it in the prepared baking dish, making sure it covers the bottom and sides evenly. Trim any excess dough hanging over the edges.
Spread the prepared fruit filling evenly over the bottom pie crust.
In a separate bowl, mix together the flour, oats, brown sugar, chopped walnuts, cinnamon, and salt. Pour in the melted butter and stir until the mixture resembles coarse crumbs. Sprinkle the streusel evenly over the fruit filling.
If desired, roll out the second pie crust and place it over the streusel topping. Trim any excess dough and crimp the edges to seal the pie.
Bake the slab pie in the preheated oven for 45-50 minutes, or until the crust is golden brown and the filling is bubbly. If the crust starts to brown too quickly, cover it loosely with aluminum foil.
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