Preheat oven to 350F. Turn pie pastry out onto a floured surface and divide into 8 pieces. Roll each portion out into a a 4-1/2-inch circle. Press each dough circle into the bottom and up the sides of 8 muffin pan cups. Fold any excess crust underneath to form a border and crimp edges. Refrigerate crust while you prepare the filling.
In a medium bowl, combine blueberries, granulated sugar, cornstarch, and lemon juice until evenly combined. Set aside for a moment.
In a separate medium bowl, combine flour, oats, brown sugar, cinnamon, a pinch of salt, and melted butter until crumbly.
Divide blueberry filling between crusts and top with oat streusel. Bake tarts at 350F 35-40 minutes until the filling is bubbly and topping is golden brown.
Allow tarts to cool completely in pan on a wire cooling rack. Once cooled, remove tarts from pan with a butter knife and serve with whipped cream or vanilla ice cream. Enjoy!