Preheat oven to 375 degrees and line a 13x9-inch baking pan with heavy-duty aluminum foil.
In a large bowl, combine flour, oats, 1/2 cup sugar, brown sugar, baking powder, cinnamon, nutmeg and salt.
Add egg and scatter butter across the top. Cut butter in with a pastry blender until crumbly.
Press half of flour mixture into prepared pan.
In a medium bowl, stir together the remaining 1/2 cup of sugar and cornstarch. Stir in blueberries and lemon juice.
Spoon blueberry mixture over prepared crust.
Crumble remaining flour mixture on top of blueberries.
Bake 40 to 45 minutes or until lightly browned. Let cool completely before cutting into squares.