1. Preheat the oven to 350° Fahrenheit (176° Celsius) and grease the bottom and sides of an 8 inch springform pan. Set aside.
2. Make the healthy crust by stirring together the oats, flour, sweetener, and salt for the crust. Use a fork or pastry cutter to cut in softened butter, oil, or applesauce.
3. Press dough very firmly into the bottom of the pan. Bake 12 minutes, then remove and let cool.
4. Make the filling by whisking the blueberries, water, salt, cornstarch, optional lemon zest, and sweetener for the filling in a bowl until well mixed. Spread this filling over the crust.
5. Finally, make the streusel for the healthy blueberry pie by stirring together the remaining flour, sweetener, cinnamon, and oats. Cut in the butter, oil, or applesauce to form small crumbles.
6. Sprinkle the oat crumble over the pie, then place the pan back in the oven. Bake 50 minutes or until crust is golden and blueberry filling is thick and bubbly. (If crumbles brown too quickly and filling is not yet bubbling, cover the top of the pan loosely with tin foil or a cookie sheet and continue to bake.)
7. Remove the pie from the oven and let cool before slicing and serving. This healthy dessert is delicious hot or cold.