Put blueberries in a large, heavy bottomed pot. Add enough sugar to make the berries sweet to your taste. For sweet, ripe berries about 1/2 cup of sugar is my preference. Omit if desired.
Mix cornstarch and water in a small bowl until no lumps remain. Add cornstarch mixture, and salt to the blueberries. Stir to combine. Add the chunk of butter and place pot over medium heat. Bring the mixture just to a boil stirring often enough to prevent burning. It will be done about the time you are done making the crust.
To Make The Crust
Preheat oven to 400 F.
Combine flour, baking powder, salt, and sugar in the food processor and pulse to blend. Add pats of cold butter and pulse until the mixture is sort of a coarse consistency, no chunks bigger than small peas.
Dump contents of blender into larger mixing bowl. Run your fingers through to feel for any large chunks the food processor missed. Smash them to smaller pieces with your fingers.
Pour boiling water over flour mixture. Stir with a fork just until a crumbly dough forms. Make sure to get the dregs from the bottom.
When filling comes to a boil, pour into a 10.5" x 15" glass casserole dish*. Spread the crust dough evenly over the fruit while it is still hot. Bake in preheated oven for 25-30 minutes until the top crust is golden. It is best to use a fork and lift a bit of crust in the middle to check and make sure the bottom of the filling is not doughy.
Let cool for a few minutes and serve warm with ice cream or homemade whipped cream.
* If you are using a different size or shape pan the cooking time may need to be adjusted. For instance, a 2.5 quart oval casserole dish takes about 45 minutes.