Filling: Whisk together sugar, flour, cinnamon, nutmeg and salt. Pour over plums and stir gently until all fruit is evenly coated.
Streusel Topping: In a medium bowl mix 1/2 cup flour, brown sugar, 1/2 teaspoon cinnamon, 1/4 teaspoon and salt. Work in butter with fingers until all ingredients are well combined. Set aside.
Pour filling into pie shell. Evenly cover fruit with streusel topping and bake in 375 oven for 45 to 55 minutes. Serve warm, or at room temperature.