Preheat oven to 350 F.

Combine blueberries, sugar substitute, vanilla, lemon zest and water in a small saucepan. Bring to boil and simmer, stirring, until the sweetener is dissolved.

Mix cornstarch with 1 Tbsp water and add to blueberries mixture. Bring to boil. Cook until thickened, about 2 minutes. Pour into lightly buttered pie dish (or 4 ramekins for individual servings) and cover with topping.

TOPPING: Mix flour, baking powder, salt, sugar substitute and butter in a food processor or by hand until the mixture is crumbly. Add milk and process until the mixture just holds together.

Using your hands, shape small pieces of dough and lay them on top of the filling.

Bake until golden, about 30 minutes.