How to Make It
Preheat oven to 350°F. Line a rimmed baking sheet with parchment paper. Process pistachios in a food processor until roughly chopped. Add flour, sugar, and salt; process just until combined. Add butter, and process until mixture is thoroughly combined and forms pea-size crumbs.
Spread mixture on prepared baking sheet. Press gently with damp hands, flattening crumble to 1/2 inch thick. Bake in preheated oven until crumble starts to brown at edges, about 15 minutes. Remove from oven, and, using a flat spatula, turn crumble, in pieces. Return to oven, and bake until crumble is golden brown, 8 to 10 minutes. Transfer to a wire rack, and cool on baking sheet completely, about 20 minutes, before roughly crumbling.