Preheat the oven to 375 degrees F (190 degrees C).

Combine plums, sugar, flour, lemon juice, salt, and cinnamon in a bowl. Pour into the pastry shell.

Combine sugar, flour, cinnamon, and nutmeg in a small bowl. Cut in butter until topping mixture resembles coarse crumbs. Sprinkle over plum mixture.

Bake in the preheated oven until golden brown, 50 to 60 minutes.