Preheat the oven to 375°F and line an 8x8-inch square pan with parchment paper, or lightly coat with nonstick spray.
Crust
In a food processor, pulse together flour, powdered sugar, and salt. Cut butter into pieces and add to the food processor. Pulse until the mixture starts to look like wet sand. Pulse in the egg until dough comes together uniformly.
Dump into the prepared pan and press into an even layer. Prick with a fork and chill for 10 minutes. While chilling, make the filling.
Filling
In a small saucepan over medium heat, melt the butter. Once melted, add the blueberries, granulated sugar, and lemon juice. Stir to coat and cook 5-10 minutes, until blueberries are releasing their juices, sugar has dissolved, and mixture is thickening somewhat. Remove from heat and let cool.
Bake the crust
After the crust has chilled, place in the oven and bake for 8-10 minutes. Remove from the oven and let cool slightly.
Topping
In a bowl, mix together the flour, granulated sugar, and butter with a fork or your fingers until uniformly mixed and forming into small clumps. There should be no loose dry bits.
Assemble
Pour the cooled filling on top of the crust, making sure it reaches all of the edges. Scatter on the crumble pieces so they cover the topping evenly. You will have gaps; if you prefer a full coating, double the topping amount.
Place in the oven and bake for 30 minutes, until the edges are bubbly and crumb topping is golden.
Let cool before removing from the pan, slicing, and serving.
Notes
If you use frozen blueberries, make sure they are thawed and well-drained.
If you prefer to not cook the filling, omit the 2 tablespoons butter from the filling ingredients and simply toss together the blueberries, sugar, and lemon juice.